To be clear and towards the OP's comment about ESP32 ISA -- Xtensa isn't really a self contained architecture, it can be customized (extended) by the vendor. The ISA can be extended for these customizations. ESP32 is one customization of it.
It is a cool design, but it was a major PITA for awhile. Xtensa is parametric so every instance of a cpu has a custom instruction set.