Master pages in Dorico allow the same formatting to be applied to different pages in different layouts.
Page size and margins, page orientation, and staff size for full score and part layouts are specified in the layout options.
All pages of your score and part layouts inherit their format from master pages. Whenever you create or change anything on master pages, this is automatically reflected on the pages that use these master pages. For example, if you insert a new frame on a master page, a corresponding frame appears on all the pages that use it.
In Dorico, master pages consist of page pairs. Every page pair has a left and a right master page. This means that if a page in your notation falls on a left page, the formatting of the left page of the page pair is used.
Dorico provides default master pages with page pairs for first (First) and subsequent (Default) pages. This enables you to have different formatting on first and subsequent pages. The page pairs are contained in master page sets for full score and part layouts. The master page sets are automatically applied for every score that you create.
If you want to change the master page sets or to change the format of the master pages, you can either create new master pages or master page sets, edit the default master pages in the master page sets using the master page editor, or override individual master pages.