The 32-bit to 64-bit ARM transition was a huge deal. It was made somewhat seamless to end users because both copies of the entire system stack could be included

The 32-bit to 64-bit ARM transition was a huge deal. It was made somewhat seamless to end users because both copies of the entire system stack could be included for a few releases, but it was at least as big a deal as the 32-to-64-bit x86 transition. Moreover, it came with the Secure Enclave, which totally changed the security model for iOS devices. T2 Macs essentially also moved to the iOS Secure Boot model. There is still an UEFI firmware but it lives in the coprocessor now, and it doesn't do as much as it used to. Many of the things used to run in the UEFI firmware now run in the T2.