I'll try to make a short summary here.. this isn't everything / this is kinda just scratching the surface to understand the entire concept, but-

So as a whole, Critters are supposed to be little cartoon-based creatures that, depending on the type of critter (Hues = the colorful critters, Blanks = the white / off white critters, and Keys = the black goopy shapeshifters which I wont get into here) -can code little universes, represented by white cubes. The cubes are a required context I have to ramble about before I can really get into the critters themselves? So bare with me lol.
"Adding data" to a cube works anomalously?? Like the Creator-Hue just has to hold it and think about all the details they want to project into it.
The cubes themselves (like, the base / shell) are not made by the Hues, they have to come in possession of one to do all this, but:

The cubes can be Alternate Universes, or Pocket Dimensions, difference really being..

Alternate Universes:
-They are based on their own realm, and will be something like "everyone is this species" or "this event was changed in the timeline" etc etc.
-Every critter alive would automatically be generated into an AU, even other critters the creator doesn't know exists.
-They are majorly self-developing and always have a timeline which, when peering into an AU cube, can be watched from any point in it's timeline.
-In AUs, when entered (via phasing through the cube), you will appear in the form of your equivalent and be placed wherever they are in the universe (so if it's the deer AU and canon Hadel entered it, he's suddenly in the body of his equivalent, popping in the middle of whatever actual deer Hadel was doing before canon Hadel entered).
-You will retain your memories from your real realm, but your AU equivalent will simply "blank out" and not remember anything you did in their body.
-Popping back out of an AU is easy, you just have to mentally project yourself, basically (this is so the AU equivalent still exists in a less jarring state after you're out).

Pocket Universes / Pocket Dimensions:
-A completely blank slate.
-A lot more private. Can be coded so no one but the creator can peer into them.
-A pocket dimension has no limits, but doesn't auto-populate inhabitants either. You can make up new species here, a cast of fictional characters, just make it a peaceful, quiet realm to escape to with no inhabitants, generate items and stuff within it, etc etc.
-You can still alter the body of existing critters (including your own) in these too if they happen to be entered by said critters, but the creator has to specifically have that idea in mind and it is usually though on an individual basis (so Luca could purposefully code his pocket universe to make Hadel have a dick if he enters it, for example).
-These are harder to escape, because while entering ANY cube involves you phasing through it in the outer / real realm, getting out of a pocket dimension requires you to phase back through it again rather than just mentally projecting back out, so you have to keep the exit cube on you or memorize where it is.
-Similarly because you're not just projected into your equivalent's body when phasing through it and rather exit out of another "exit cube" within the pocket universe, it can be particularly dangerous for Blanks to enter these because they can be set up as death traps for them (imagine jumping into a cube and you fall 80 ft into a pool of endless razors for example).
-These usually don't have a timeline unless you fill it with your own characters or specifically code it to have one. A lot of these are used as still, quiet, peaceful realms to escape to during stress, but other Hues like to basically make TV shows out of them.
-You CAN make it self developing if you want it to be, but you have to give the cube things to work off of, like the inhabitants it's filled with, personalities for them, some type of plot device, etc etc. You can budge in and change/redirect anything from any point in it's timeline at any time.
-The inner universe can come in any "style" (be realistic, look like a 3D low poly PS1 game, look pixelated, etc etc.), however unless you code specific critters to fit the design of the world, a critter entering it will retain their canon cartoon style.

Okay now, summary of the types:

Hues:
-Colorful, sometimes have markings
-Omnivorous
-Can add data to cubes / claim them
-Can enter their OWN cube, but cannot take anything originating from them out of them.
-Like 99.99% of the population
-Have animal-like traits such as purring, hissing, growling, becoming territorial or possessive of mates, going into heat or rut, having heightened senses to various degrees, emitting pheromones, and scent-marking (their mates, especially in rut).
-Have cartoon-like traits such as physical 2-D features, cartoonish movements / body language, rapid healing properties (parodying a character in a cartoon show being majorly injured and then fine in the next episode), resistance to death (parodying cartoon pain humor, they CAN still die though), and agelessness (parodying 20 season cartoons where characters never age; their "physical age cap" is completely random, some critters will stop aging at 15, some at 30, some at 80, etc.)

Blanks:
-White or off-white (pure white Blanks are called "pure breds", they only happen if both parents are Blank (even if both are off-white)). Have no markings / are solid colored.
-Lack the ability to claim or add data to a cube.
-Can enter ANYONE'S cube, and take things and other Critters in and out of them freely.
-Are nearly extinct.
-Have all the animal and cartoon traits as Hues do
-Additionally have "feralism" traits, which are triggered episodes based on fear, anger, or mania where the Blank gains a disassociative and animalistic mind. If triggered by fear or anger they can become super aggressive and unconscious / out of control, if triggered by mania, its more just like.. a dog with zoomies (also, if a Blank with zoomies is around other Blanks, it often triggers the other Blanks to snap into zoomies too).
-All have super sharp, pointy teeth and the ability to inject venom in their bite. This is controllable UNLESS they are feral.
-Carnivorous only, chocolate and dairy can make them sick, but their bodies are resistant to parasites and diseases raw meat or spoiled food can carry.
-Can be extremely food-possessive when starved. Placing a slab of meat in front of a starving Blank can cause them to go feral and attack others hovering around them and the food.
-Considered dangerous in their society, like animals that you need to be constantly wary around (however this is majorly untrue and a fear tactic by those leading their society. Blanks rarely go feral out of aggression even when intimidated or afraid. The more traumatized a Blank is, the easier it is to trigger their feralism (like Hadel gets triggered all the time), but there are many Blanks who have never gone feral their entire lives).

Part of the deeper lore of why Blanks are both so rare and why they are so needlessly feared is because of their interactive ability with the cubes that Hues lack and events or propaganda pushed centuries ago that, at this point, is mindlessly still subtly followed. The people at the top don't like them because they fear Hues cooperating with Blanks can be used to overthrow them, so a long ass time ago, Blanks mysteriously started disappearing en masse, and the ones remaining had their biology used against them to be feared and degraded within their society to keep them in place and keep Blanks and Hues from harmonizing.