not at all!! i think there are very clear lines of plagiarism, so i’ll tell u some things that i personally avoid: copying things word for word, copying the plot structure for a fic,
for ex: if someone reads ur fic and says that they see very big similarities with another fic (as in, the scenes that happen in Fic A are in the exact same order but worded differently as Fic B’s)...

things i DON’T consider plagiarism is copying a certain trope or personality of a character that someone else has written, because i think it’s something you’ll see in every fic like that kind of thing is pretty normal to me!! if there’s a scene you rlly like in a fic and it inspires u to write something surrounding that id say that’s perfectly fine? it just depends on how much you actually take from it cos that can be harmful!! number one thing to rmr is that if u get that similar feeling of reading an online article and “rephrasing” it for an essay ur about to hand in to your professor... you’re plagiarizing!!

that’s just my opinion, but you can always tell me about specifics of what you want to do and we can go from there