How are posts in a category fetched from the database?

I would like to know how the posts belonging to a certain category are fetched from the database. It seems the database is filtered by category slug. I’ve got a website where I need it to be filtered by Category name (so, if I want to make category pages, I want the posts displayed there to be selected from the database based on either the name or the ID of the category, not the slug).

I don’t know in which file to make this change and I’m not very sure how to do it either. I would appreciate your help very much.