Page layout assignment by profile and record type
Key Takeaway
The user's profile picks the page layout. When an object uses record types, the profile and the record's record type pick it together. Page layout assignments live only in profiles, never in permission sets.
A page layout controls which fields, buttons, and related lists users see on a record page. After you build page layouts, you assign them so each user sees the right one.
What decides which page layout a user sees
A user's profile determines which page layout they see for an object. If the object uses record types, the combination of the user's profile and the record's record type decides which page layout appears. Two profiles can therefore see different page layouts for the same record type.
For example, imagine the Opportunity object has a Renewal record type, and your org has a Marketing profile and a Finance profile. For the Renewal record type, you assign one page layout to the Marketing profile and a different page layout to the Finance profile. When a Marketing user and a Finance user open the same Renewal opportunity, each one sees the page layout their own profile assigns to that record type.
Master is a system-generated record type. A record uses Master when it has no custom record type. Every other record type is a custom record type.