This is accomplished in 2 stages in Scrivener version 3.
First stage: Make sure your folders & documents have appropriate section types assigned to them. You can do this manually, but it’s a lot less tedious if you make Scrivener assign them automatically, and only change them where there are exceptions.
Second stage, when you bring up the compile window, using the center pane to assign formats to each of the section types of your folders & documents. If you chose a novel compile preset (in the left-most pane), there should be formats that include PART X headers on their own pages as a choice.
If that’s all gibberish to you, you may want to check out the sections of the tutorial (under help) explaining section types and the compile process. Or check out this explanation on the blog, which goes into detail about how compile changed from version 2 to version 3: literatureandlatte.com/blog … ng-compile