I want to override results of an autocomplete field (Entity reference in a content)
It's pretty much the same as Show node IDs along with titles in Entity reference Autocomplete list except you use content type name instead of node id. 

Currently there is a bug in Drupal 8 where this does not work properly. Here is the issue: Views Filter by an entity reference view not working as expected you could try the patch in comment #26 or the module in comment #13. I have not tried them yet.