I'm pretty confused by all this! Ie why they are set up like this. Example: All STM32 dev boards have two USB ports: One connected to the USB periph; one to a built-in ST-Link (JTAG). You flash and debug/print to CLI off JTAG, and use the USB one if your device needs to communicate with a PC etc during operation, or if you want to use DFU flashing for production boards or firmware updates.

If you are designing a board, you will probably always have the JTAG pins broken out to a port of your choice, and use an external debugger. Wire USB A/R.

The USB dev boards for C3 seem to all have only a UART bridge USB, and no JTAG! That is confusing because A: I'm not sure if this is a full-up USB peripheral for use as serial (But maybe not HID? Maybe it presents as USB-serial to the PC, but you program it on the MCU like UART?), and B: Why JTAG isn't table-stakes for a dev board.

I ended up buying a "C3-Rust" devboard, because it was the only one I found that had JTAG USB! (I am coincidentally using Rust, but that's only superficially relevant)