Second-stage bootloader
The small go-between the boot code loads first, because the real bootloader is too big to fit where the system wakes up.
It's a chicken-and-egg problem. When an SoC powers on there is only the boot ROM and a few hundred kilobytes of internal SRAM — DRAM doesn't work until someone configures its timing, and whatever does that won't fit in SRAM if it also has to read filesystems and talk to networks. The answer is an intermediate stage. U-Boot calls it SPL, the Secondary Program Loader: a stripped-down build that only brings up RAM and loads full-size U-Boot into it.
On the PC side the MBR did the same with 446 bytes of code, which is why GRUB long had stages 1, 1.5 and 2. The RP2040 offers an unusually tidy miniature: the boot ROM reads a 256-byte "boot2" from flash, checks its CRC32, and boot2 then configures the flash chip for execute in place.