Google takes the bundle, and builds a bunch of binaries for different architectures & device OS versions. The changes aren't subtle, they delete things the user

Google takes the bundle, and builds a bunch of binaries for different architectures & device OS versions. The changes aren't subtle, they delete things the user's device doesn't need and sign the result. E.g. if the user has a 32-bit ARM device they'll delete 64-bit ARM code from the bundle when building the binary, and likewise for the reverse. That saves users both mobile data and storage space.