How to find if apk is 32-bit or 64-bit

APK is ZIP. You can open it and check directory lib to see which architectures are supported. If there is no directory lib, it supports all architectures.

64-bit Android is backwards compatible and can run 32-bit applications. On the other hand, Intel Android (both 32-bit and 64-bit) has very limited compatibility with ARM Android (a binary translator called libhoudini is shipped on Intel but from my experience, it works for only very simple code), and ARM Android is completely incompatible with Intel Android. This affects only applications with NDK code in lib, pure Java applications can be run on both.