Before loading UEFI the Trusted Firmware is booted. What is trusted firmware?
Think of it as basically the first stage open source secure bootloader for ARMv8-A platforms. It’s a reference implementation by ARM itself adapted by most if not all ARMv8-A SoC vendors. It’s more than just a bootloader though since it allows utilization of TrustZone technology by e.g. handling world switching, providing standard interfaces for PSCI, secure boot, authentication framework, etc etc. See https://github.com/ARM-software/arm-trusted-firmware and https://www.slideshare.net/mobile/linaroorg/arm-trusted-firmareforarmv8alcu13 for more details.
@vchong thanks for your easy to understand description and the links.