They’re XML, structured into dictionaries and arrays containing key-value pairs. Preference plists are managed, and need special treatment.
Preference files are involved in many Mac problems, and are often their solution. Using them now isn’t simple: here is some guidance.
The current macOS implementation of preferences is appalling design in every respect, and is essentially undocumented even for developers. Is this the way that macOS is heading?
Have you ever tried removing or editing a preference file, to no effect? This explains why that happens, and how to get changes to work properly.