Yes, the boot picker screen is in firmware, so similar to BIOS.
I am pretty sure that the boot picker is supported by the Secure Enclave chip in your m1 mini.
It would use the boot files that are on a bootable partition, so, in that sense, the boot loader is part of any bootable partition. Maybe this is not technically accurate, but certainly close enough, as there has to be some kind of firmware support for any bootable device. And, the operating system has to provide boot files that use that firmware. All kinda works together, right?
And, for the fusion drive. That is actually not a real drive at all. It is a software configuration of the two devices (spinning hard drive, and the m.2-type blade that is the SSD in the fusion drive). The two devices are combined into one virtual drive, similar in some ways to a RAID. And, you don't have the hard drive (replaced with your SSD), and you erased the left-over SSD, so now you only have a configuration setup, but nothing still exists on the fusion drive, it's just a "software shadow". You COULD choose that drive in the list on your Disk Utility, then right-click on whatever listing for the fusion drive, then choose delete, or remove, or simply unmount.