Hi,
Chap 16 of the Writer Guide has everything about master documents.
But it may be a bit confusing. A picture on MD link structure is important and another picture on the styles precedence between MD and chapters is needed.
In my experience, the most important rule is to use the same template for MD and chapters. To be more precise, MD and chapters must share the exact same styles names, so MD styles attributes overwrites/supersedes chapter styles attributes.
The second most important rule is to use the template styles only, no spurious. left over legacy styles. And avoid direct formatting, manual breaks, and other typewriter-ways of formatting contents (indentation by spaces, empty paragraphs, etc...)
Failing to follow these 2 rules and the master document is just more work for a single person editor.
Olivier