Stereotypes can form a few ways. Not all of which are based upon truth. Illusory correlation is the phenomenon of perceiving a relationship between variables even when no such relationship exists.

/s ?

No I think some of them are just racism.

Every single stereotype is based in truth propaganda.

Most stereotypes have their birth in cultural propaganda. They reinforce the tribal in-group vs out-group that humans naturally fall into.

A lot of stereotypes are based on pure racism and general prejudice