OK thanks. I imagined the distinction to be that a BIOS functions as an API and abstraction layer over the hardware, without preventing the OS from accessing it

OK thanks. I imagined the distinction to be that a BIOS functions as an API and abstraction layer over the hardware, without preventing the OS from accessing it directly, whereas a hypervisor functions as an isolation layer, preventing an OS from direct access, and also thereby facilitating simultaneous hosting of multiple OSes on the same hardware.