With SWD and a Tag-Connect connector[1]. I watched someone on youtube recommend it due to the small space and low profile. You can use the VSCode NRF Connect SDK plugin (coupled with Jlink or NRF52 dev kit) to flash the device. But of course you can just use USB und copy a generated .hex file directly on the device. I use SWD because its faster to flash many boards by just pressing the pogo pins on the connector pad. Boom done. :)

[1] https://www.tag-connect.com/product/tc2030-ctx-nl-6-pin-no-l...