Most confusion about layouts comes from learning the pieces separately. The chain from input type to template draws the five links once, says where each is defined, and explains why a new page type starts in the admin rather than in the repository. Every later unit of this guide is about one of those links.
Where content is stored, and what a change does to it shows where content lives (in the record's revisions, read from the active one) and predicts what a rename, a layout switch, a type change, hiding and deleting do to existing content. Read it before you change a layout that authors have already filled in.