Execute in place (XIP)
The CPU fetches instructions straight from flash instead of copying the program into RAM first.
On a microcontroller with 64 KB of RAM and 512 KB of flash there is no choice: the program runs where it lives. That needs memory that can be read randomly, byte by byte, inside the address space — which NOR flash can do and NAND cannot. Hence NOR in nearly every microcontroller, while page-oriented NAND always has to be copied to RAM before anything runs from it.
External flash on QSPI can be mapped into the address space as well, usually behind a small cache to hide the fact that every miss costs dozens of cycles. The Raspberry Pi Pico runs its entire program that way. The catch is that you can't write to flash while executing from it — anything that updates firmware or saves settings has to move the routine to RAM first, or the CPU goes to fetch its next instruction from a memory that is momentarily not answering.