Fan art

It shows up once an audience feels ownership of something in the work, which is why it clusters around characters, catchphrases and recurring bits more than around information. Creators tend to feed the loop deliberately: a dedicated hashtag, a corner of the community tab, a slot at the end of a video, occasionally a channel banner. Two things to get right. Credit the artist by name every single time, and check before putting anything on merchandise, because the artist owns their drawing whatever it happens to depict.