I've never used a stand-alone one. I've got Arduino sketches I can quickly flash onto an Uno (or my 2016 HiFive1 320 MHz RISC-V board, which is usually closer

I've never used a stand-alone one. I've got Arduino sketches I can quickly flash onto an Uno (or my 2016 HiFive1 320 MHz RISC-V board, which is usually closer to hand) to do all manner of things including UART, flashing a bare AVR chip e.g. ATTiny85 (now supplanted by CH32V003) and so forth. Same principle as using a Pico.