Can SwiftUI 2.0 new features introduced in WWDC 2020 run on iOS 13?

Let's say I compile my app in Xcode 12 and add these new codes, will it be compatible with iOS 13 devices?

No, it will require deployment target to be set to iOS 14.

Or do i have to keep my existing code, and use the new one only if the user is running iOS 14?

Yes, you will need to add new features conditionally using availability checkers.

According to wwdc video SwiftUI 2.0 is for iOS 14 it mean the deployment target for app should be set to iOS 14. Apple says that iOS 14 can run on the iPhone 6s and later, which is the exact same as iOS 13. This means that any iPhone supported by iOS 13 is also supported by iOS 14. So without any second thought you can go ahead with new features of SwiftUI