What's the principle of Bitcode in Xcode?

Bitcode is an intermediate representation of a compiled program.

Meaning you can slice/segment it out for different processors, etc. once it's in the store, reducing the payload do the user accordingly.

Providing Bitcode with your build allows Apple to recomplie your application to optimize for new devices or iOS versions without you having to provide a new build of your application.

Including Bitcode is optional for iOS apps, but mandatory for watchOS or tvOS apps.

"Meaning you can slice/segment it out for different processors, etc. once it's in the store, reducing the payload do the user accordingly."

That's Slicing, not Bitcode.

They work together towards thinning, where, as the docs say, the three components of app thinning are: slicing, bitcode, and on-demand resources