First Are you using bluebuild? There a many forks of custom image starting point. Secondly if your usijg bluebuild template. All files that specify packages and what modules should be on the recipes folder. Using recipe.yml is a very good starting point as you can specify how to install a package and what package manger to use. IE brew, rpm-ostree
You can also add modles which you can take the bluebuild template and add a modules folder in the main tree. Bluebuild has premade modules, aswell as documentation to make a cutom module.
And to answer your first question directly use recipe.yml in the recipes folder to specify other .yml files to be used in building. Secondly the best you can get is githubs builder in the actions tab, however some errors are BLOODY USELESS BECAUSE THERE WRONG. I have had a few times where it complained about not having a - at the top of a module. Even though there was one there, i eventually solved it after rewriting a part of my .yml i belive it was a formating issue