Brain-wise, it’s a bare nRF52840 chip along with a TI BQ24075 for battery charging. The interesting thing about this implementation is that [ebastler] used and abused Nordic sample schematic #4 ...

There’s really not much to this build, which relies on the Adafruit Feather nRF52840 and will readily ... adapter clicks on to the connector. The schematic, code, and STL files are all there ...