UML runs only on Linux and only on x86, amd64 and powerpc. Which is a real shame, otherwise you could run a full Linux kernel on all these arm Android devices.

A shame, true, imagine having a whole Linux kernel as a macOS binary, running Linux containers with as little overhead as possible.

UML and "as little overhead as possible" probably shouldn't appear in the same train of thought. I remember it from the very earliest Linux VPS providers, IIRC it only got semi-usable with some custom work (google "skas3 patch"), prior to which it depended on a single process calling ptrace() on all the usermode processes to implement the containerization. And there's another keyword that should never appear alongside low overhead in the same train of thought

Page-grained mappings UML does make for tons of overhead. AFAIR Linux even considered a specialized reverse page mapping structure just to accelerate those, but ultimately dropped it because of memory overhead and code complexity.

Realistically, the overhead isn't ever going to be lower than hardware virtualization unless one goes for an API proxy a-la wine and WSL1 - but that's tons of work.