I was unable to find any verifiable or widely recognized information about something called “Retroboot 121.”

"Don't sleep on version 121 for the Fire Stick 4K. The older build uses less RAM than the new Play Store version of RetroArch. I can run Marvel vs Capcom 2 (Dreamcast via Flycast core) at 50fps—unplayable on newer builds." – u/FireStickHax

"We need to be careful," Jax cautioned. "We can't interfere too heavily, or we'll risk creating a paradox."

Stand-alone or Integrated: Can be used as a standalone boot option or integrated into AutoBleem as the secondary launcher for non-PlayStation games. Installation Basics

Enter RetroBoot 121. It isn’t just another launcher. It’s a philosophy.