You have an ESP8266 board like the Wemos D1, a Li-Ion battery ... providing charging and battery protection features, a jumper, and maybe a pushbutton – the Fritzing diagram shows everything ...

So they decided to base the guts on common components that can be put together easily, specifically a Wemos Mini D1 with an OLED shield as a bright display. They also designed a cool tiltable 3D ...