Page layout assignment by profile and record type
Key Takeaway
A user's profile picks the page layout, together with the record's record type when the object uses record types. Page layout assignments live in profiles only, never in permission sets.
A page layout controls which fields, buttons, related lists, and other elements users see on a record. Page layout assignment decides which page layout each user gets.
How the profile and record type pick the page layout
A user's profile determines which page layout they see. If the object uses record types, the page layout shown on a record depends on both the user's profile and that record's record type.
You assign page layouts in the assignment table. Without record types, the table has one page layout choice per profile. With record types, it becomes a matrix, with a page layout selector for each profile and record type pair. The page layout you choose for a pair controls what that profile's users see when they create records with that record type.
For example, imagine the Opportunity object has two record types, New Business and Renewal, and your org has two profiles, Sales and Support. The matrix has a selector for each pair, such as Sales and Renewal. You could choose one page layout for Sales and New Business, and a different page layout for Sales and Renewal. Users with the Sales profile would then see different fields and related lists, depending on the record type of the opportunity they open.