The confusing part of “it’s” vs “its” is that “its” is a pronoun, and therefore its possessive doesn’t use the apostrophe, where as you use one with possessive nouns. So usually when you are writing you’re thinking about the possessive relationship, not so much about whether you are using a pronoun to describe your subject.
“My Lemmy account’s username”
“Its username”
It’s funny because my phone defaults to adding the apostrophe when I just type “its” but if I follow it with a noun (or adjective) it automatically goes back and removes it.