This is a bootloader for the RP2040 which enables code upload via UART.
There's a more complete description at: https://blog.usedbytes.com/2021/12/pico-serial-bootloader/
There are currently two tools I know of which can be used to upload code to it:
- serial-flash - my tool written in
go - pico-py-serial-flash - a similar tool written in Python, contributed by another user.