It's based on ARM TrustZone tech, and is (probably amongst other things) a tpm2 implementation within the SoC that also gates silicon initialisation (the x86 cores will not spin up without it).

I'm not sure it could be thought of as equivalent to Intel Management Engine in terms of it's overall capability, however.

It's a security processor built into the main CPU. The AMD and Intel concepts are going to be similar enough that this talk will do:

• https://www.youtube.com/watch?v=lR0nh-TdpVg
• https://www.bleepingcomputer.com/news/hardware/intels-secret-cpu-on-chip-management-engine-me-runs-on-minix-os/
• https://forum.osdev.org/viewtopic.php?t=32549

MINIX is likely the most used OS on earth because it's running inside the x86 processor. tl;dr the processor presented to your operating system isn't the real core of the computer; it's just a virtual machine.

EDIT: I knew there was more, but it's been so long I'd forgotten: https://www.youtube.com/watch?v=jmTwlEh8L7g