Yes and no? In a way of course everyone is reincarnated, that's just the way of life. But i know you're asking bc of the earlier question, so the answer is that most of them are indeed reincarnated, but some like Reef, Yasumi, Ammianus and Bruno aren't. Violet is also technically not reincarnated, because she is a digital copy of a brain that was put into a body, so she was revived, similar but not the same.
Other's have partial reincarnations like Gemma and Shellie, where they aren't aware of it outside of some deju vus and 'corrupted' memories like a computer.

On how it works... i never really put any thought into it, but whether someone remembers everything from their previous life or just some parts is totally random. But with enought tries they can remember more. As for the way they're reincarnated, it's mostly because of the Alterna Liquid crystals. Ya know the thing in the Alterna logs that made marine life evolve so much in 12 thousand years? Yeah they canonically contain human desires and dreams, and in my fanfic's canon they serve as a connection between the different universe versions of everybody, so only people that contain a lot of it in their dna can actually reincarnated. For example: Jack originally wasn't reincarnated as a Salmonid, only after contact with the crystals when his family were near the splatlands did he reincarnate.
Another requisite for reincarnation is that the person has to obviously be dead or in a prolonged coma.
I hope this is a good explanation :D

Sorry for the late response, I don't really know how this site works :3
ummm I have A LOT OF OC's , at least 80 of them... So I'll just stick to my splat oc's and give a short description about each

-Cade L. Caldwell: Died as Cade Caldwell in our world, reincarnated into another of my oc's world, inside a new body and sharing it with his alternate universe Splatoon self: Livie Aoyama. They didn't get along at first, but after a few years and their eventual death as sacrifice, their minds fused together and got put into Livie's old body back in the Splatoon world. Yet she only remebers her life as Cade Caldwell. Cade, Sam and Camille after reincarnating got "adopted"(not really, more like became roommates) by Reef and started living with them.

-Camille Caldwell: Cade's younger sister, reincarnated along with her sibling and her sibling's girlfriend( well, boyfriend now, due to reincarnation stuff). There's nothing really to note about her aside from the fact that her body is also the body of the younger sister of Livie ( even in different world's they are still siblings), and that she's very anxious.

-Coraline Reef Dallimore: 4th (and least favorite) child of rich parents. Recently got kicked out of her old house into a new one in Tempura Town (oc town for the fic). She's very goofy and happy, but very hard working and serious when doing agent duties/doing schoolwork. She has a lot of issues with her family, although deep down she loves them and sometimes misses them. She made Cade, Sam and Camille into her roommates after finding them alone and homeless in some dark hallways in the middle of the night. When she became agent 4 she initially tried to hide it, but with their reincarnator knowledge the other figured it out the day it happened, and now Cade helps her by upgrading her weapons, and talking alongside Marie in the comms. She's in love with agent 8

-Sam Styx: Previously Samantha Styx, he's the Cade's partner, and the sanest of all the oc's. He met Cade when he was 8 in funeral home(? i don't really understand how they work), when his twin sister died from diseases or weak constitution ( not sure which to pick) and when Cade's mom had been murdered. They have both been inseparable since then. Sam likes to make art and be fashionable. One of his issues is that when he reincarnated into splatoon with the other 2 he was the one that took the longest to get used to everything, as his his alternate self was very skeptical, and said trait was passed onto him.

-Aaron Styx: Younger brother of Sam Styx, and Agent 7 of the Squidbeak Splatoon. He's very happy go lucky, and reincarnated as an octoling during Octo expansion. He took on the role of an older brother to Yasumi after initially meeting her in the beginning of OE, and regularly helps her in the tests. It was his idea to save Hibiki when they found her.

-Yasumi Styx: Adopted Sister of Sam and Aaron. Agent 8 of the Squidbeak Splatoon. Very curious and sassy, will not take anyone's shit, and will poke somebodies eyes out with her boots heels if need be. She's in love with Agent 4

-Hibiki: Almost fully/Semi Sanitized Octoling. Has very little knowledge from either her past human life, or her current life as an octoling due to the primordial ooze. She was found by Aaron and Yasumi while trying to fight both Tartar's control and the oncoming waves of octolings in one of tests. The ooze affected her eye's a bit as she's constantly oozing some , from her left eye. Slowly but surely, after getting to the surface and being introduced to everyone else, she's being cured (Getting the Sanitized ink extracted from her) by Cade, Marina and Aaron.

-Yellow Flame Child of the Spawning Grounds, or just Jack: Salmonid that left his home after an initial contact with Alterna's Liquid crystals turned him more humanoid and made him remember his old human life. He loves to play the bass and likes to take care of everyone around him, earning him the title of the group's Dad. Currently living with Mike

-Amanda: Octoling soldier that remembered some of her old life as a human, out of nowhere, with the side effect that she mostly forgot her octoling life. She ran away quite easily from Octarian society into inkopolis, where she met Mike and Jack and started living with them, eventually falling in love with Mike. Even if she's no longer a soldier, she rarely ever takes her neo octoling armor off, as well as her headphones and brown jacket.

-Mike Oarfish: First of the group to be reincarnated, and the only one to fully live a new life since he was reincarnated moments after being born. Very mysterious and sassy ( edgelord). A prodigy with splatterscopes. He helped Agent 3 during her story mode as a sniper hidden in the levels. He owns a heavily modified E-liter which he used in the previously mentioned story mode.
Took both Jack and Amanda as housemates the moment he found them. Never takes his hat off

-Shellie Ames: Agent 3 of the Squidbeak Splatoon. Is actually the reincarnation of Sam's deceased sister, and the actual sister of Sam's Splatoon self, even if she doesn't know about either. She's very socially awkward, so she keeps up a badass mask to hide her actual personality. Not really interested in anything other than turf war. Loves her parents. Does not know she has a missing brother.

-Violet L. Alterna: A copy of the mind of girl who lived in Alterna, (similar to Johnny from cyberpunk) she was inserted into a new body created by O.R.C.A ,2 days before the events of Splatoon 3, using her original self's DNA and octarian/octoling DNA. (Hybrid kind of)
Currently Neo Agent 3.5

• Ammianus Xavier: Inkling child of a farmer and an accountant, his biggest dream is to get the best treasure ever( THE ONE PIECE!!!!) and become the King Of Splatsville. Currently Neo Agent 3

-Bruno "lil' Buddy" Xavier: Adopted brother of Ammianus. Don't have much written about him.
(I believe i have better explanations and lore for the previous 3 somewhere in my twitter profile, if not i can always post it again)

-Gemma Aoyama: Mother of Livie/Cade and Talia/Camille. She's the one the comic i'm making is based about :D. She's the reincarnation of Cade's Mom , after she was killed, but she has no memories of it outside of deja vus and cryptic dreams. She became basically depressed after her two daughters went missing, thinking they were killed by Octavio's army. One day on the way to work she read the newspaper and almost cried in the middle of the road as she found the face of her two kids (looking slightly different due to Cade's having a different hairstyle) staring at her in an article that talked about Octarian youth found running away to the surface. As of right now she is doing her absolute best to get out of her dome and be reunited with them

That's about it ! :3 sorry for any writing mistakes there might be.