Hi there,
Here is an Update video, using the JLink that’s included(built-IN) to the Nordic Nrf52840DK it is a real development board/platform for the Nordic Nrf52840 MCU and can be used for debugging and flash reading and troubleshooting. Also picture is the connection to the Xiao Expansion board and the SWD interface for Serial Wire Debugger like the Jlink. Compared to the actual $700 Jlink-II (Was a Gift) it does all of it plus more can be updated by Drag&Drop file onto JLink Flash Drive. Also is a plug for the Product of the century for them IMO.
$9 BLE Dongle, If you even think about BLE and some of the great things you can do with it. Truly is the Best $9 dollars one can spend these days LOL 
HTH
GL 
PJ 
Phone app for the NBS is on there at the end.
Notice , No soft device or Bootloader on this Blinky example written to a Xiao Nrdf52840 BLE…
Add in a Read of the Xiao with the Jlink on the DK board.
Shows the MBR first(orange) then the softdevice (blue ver 1.40) then the rest is marked as Application(green).
notice the lack of a softdevice on the DK version of the same code. less space too.
Check out the FREE video’s for getting started by number 3 you’ll get it and want more.
Well documented , Well supported, Better silicon Period. Now an SDK that is working and getting updated and improved almost quarterly. No one is doing that, SES, ESP-IDF-Arduino-IDE, ST… none.
nordic stuff is free now. (debugging built in)
HTH