How to boot from SD but run from USB?

UEFI isn't x86 specific - Arm uses it as well.

Yes, although to note that there are typically several stages of booting. The UEFI / u-boot / etc is often the second or third stage of booting. They come after things like the ROM first-stage bootloader then the second stage bootloader loadered from flash, or similar. Sometimes boot is handled by a CPU that isn't the main one, such as a 'management' CPU or in the Pi

In the classic Pi 1, the process is:

In Pi 3 and later the ROM bootloader also knows enough to read USB sticks as well as SD cards. The early stage(s) also move to EEPROM on Pi 4 to make them easier to update.

It's possible to run another bootloader like u-boot or UEFI as your fourth stage, ie:

This is useful if you need more flexible booting than the regular boot process provides - eg you want to fetch the kernel from an NFS share, which is something not supported in the standard Pi firmware.

If the board vendor puts the firmware for stages 1-4 into EEPROM/flash, the board now supports UEFI but the boot process didn't change, it just hid the first four stages from you.

Theo