Creators can create public posts just like they can create private posts, followers-only posts, local-only posts. Instances may or may not display content to no

Creators can create public posts just like they can create private posts, followers-only posts, local-only posts. Instances may or may not display content to non-logged users. It's not up to the software, it's up to what the admins and the users want to do. But if it's open, then you most probably even have access to an RSS feed of a user's public posts, so it can even be checked from outside the browser (and the specific app)