The look & Feel packages are one of Plasma 5 new ways to simplify customization. That and better organized KCMs. Unfortunately, there's zero documentation about them or if you count this as documentation: https://community.kde.org/Plasma/lookAndFeelPackage - there's that.

So there's no way of knowing what goes where, what does what although it may seem obvious to some. Documentation should be important too when implementing new features, specially if we want users to use them.

KaOS made there own Look & Feel package midna (https://github.com/KaOSx/midna) so maybe we can look in the git repository how the Look & Feel package would work. Where are the breeze and breeze-dark look & feel package is located (repository)?

I.. don't know but they are installed in /usr/share/plasma/look-and-feel. they are QML files and some content files that seem to tell Plasma what to use. But that's as much as I can tell right now.