Database design challenge

I find the hardest to do is decide on how to structure the database.

For example, I'm never quite sure when tables should have columns on both tables pointing to each other, or only one pointing to the other.

A User has a Post and it links to the Post table record, but should a Post have a Created_by column, pointing to the User? Is there a reason why to have it one way or another? Does it depend on how the front end is built or is able to query the database? (like Draftbit vs Bubble vs Webflow)

I've tried to read articles about this but i found nothing coherent and clear.

Does the way Xano is built inform how i should build a DB in xano vs elsewhere?

Any good guides (video preferably) or general suggestions on this?