A major and the most significant approach to UEFI BIOS security is to prevent it from being illegitimately modified and the SPI flash memory from being overwritten. Modern vendors use a wide range of security mechanisms to ensure that (SMM BLE / SMM BWP / PRx / Intel BIOS Guard) and hardware-supported verification technologies (Intel Boot Guard). In other words, they do everything just not to let an attacker place a rootkit into a system.
In this talk, there were some thoughts on how vendors manage to throw all those security flaws together in one system using Intel NUC, a small home PC, as an example. Besides, researchers demonstrated how an adversary can compromise BIOS from the userland.