I'm not an expert, but I do work with them a lot. It's my understanding that the old school BIOS is a completely different environment compared to the more modern UEFI and UEFI does not require any underlying BIOS to function. It's a one stop shop so to speak. Essentially, it makes use of more onboard memory then the traditional Legacy systems did to support the fancy GUI's and also has a number of features which it uses to do things like speed up boot time, but using parallel booting techniques along with tools that make restoring a bricked motherboard smoother if not down right easy, just to name a few of the benefits. But the TLDR version, no, UEFI does not run on top of an old legacy BIOS.

I'm not an expert, but I do work with them a lot. It's my understanding that the old school BIOS is a completely different environment compared to the more modern UEFI and UEFI does not require any underlying BIOS to function. It's a one stop shop so to speak. Essentially, it makes use of more onboard memory then the traditional Legacy systems did to support the fancy GUI's and also has a number of features which it uses to do things like speed up boot time, by using parallel booting techniques along with tools that make restoring a bricked motherboard smoother if not down right easy, just to name a few of the benefits. But the TLDR version, no, UEFI does not run on top of an old legacy BIOS.