A critical distinction is that it's not an emulator. "Compatibility layer" is a great description of what's actually happening. It's a (set of) native library

A critical distinction is that it's not an emulator. "Compatibility layer" is a great description of what's actually happening. It's a (set of) native library(ies) which provide the ABI and call targets that are expected by the software and wrap around native OS functions to provide those. To use a metaphor, it's like someone made a replacement edge for a puzzle, which interlocks with a subset of existing pieces for another puzzle rather than someone making a table-sandbox within which to use the the entire initial puzzle.