Xcode 27 and build SDKs

Please help me understand how build SDKs work in Xcode 27.

Let's say I have an older app that uses UIDesignRequiresCompatibility. When I

a) select an iOS 26 Simulator in Xcode 27 and press "play" (build an run), the app shows up in design compatibility mode.

b) select an iOS 27 Simulator in Xcode 27 and press "play" (build an run), the app shows up ignoring design compatibility mode.

My questions:

1. Does that mean a) is using iOS 26 as build SDK and b) is using iOS 27 as build SDK?
2. What would happen if running an app built using iOS 26 build SDK on iOS 27?
3. What would happen if running an app built using iOS 27 build SDK on iOS 26?
4. How can 2. and 3. be done with the simulator?
5. When I submit to App Store Connect, which build SDK is used?