I see lots of good usb examples in the pack installer, but is there a bootlaoder for usb example anywhere?
DFU?
wow, that will do.
Hard to say, it's the keyword you're looking for.
USB centric ST parts typically contain a ROM bootloader supporting DFU mode, and the Cube/HAL examples likely include a DFU loader. The code in ROM is like any other visible to the CPU, you can jump into it.