My understanding is that Apple hired a lot of the staff of Transitive, and had a more or less do whatever you want license with source access. Ad that the AOT mode is based on LLVM, but the JIT piece is still pretty core to the design (hence why other JITs run well on top of it).
It’s not based on LLVM, that’s way too heavy for the use case.
The same backend is used for AoT and JIT compilation. It uses a custom lightweight IR that’s really close to x86 itself, with a big focus towards reducing translation times.
Even the company that made QuickTransit is gone now, having been bought out by IBM a decade ago.