I think there are three main categories:
1.) Characters who shippers think would be cute/hot together purely based on visuals or very base personality, this includes many crack ships with characters who have never interacted.
2.) Characters who shippers think would be cute/hot together based on visuals, personalities, and their interactions in the show.
3.) Characters who shippers believe would actually be good and make sense together in canon.
I'm mainly a 2 kind of person. I love fantasy about two characters who have a great dynamic that I find attractive, but I don't necessarily need or even want them to have a canon relationship.