How compatible is Apple silicon with Intel?

Apple Silicon and Intel refer to the CPUs A CPU does not really have any compatibility with another CPU.

If you mean compatibility with software meant for Intel Macs, then Apple made Rosetta 2 which translates software meant or Intel CPUs so it can run on Apple Silicon. Most 64 Bit intel software can be run using Rosetta 2 on Apple Silicon.

See here: link-> If you need to install Rosetta on your Mac - Apple Support

They are two separate and distinct. They are basically incompatible with each other but that's not a problem as they aren't both in the same Mac.