Opening up the headphone, we can see the main PCB, Antenna, and Battery

Battery is Li-ion 3.7V / 0.32Wh or 3.7V / 86mAh. The battery appears to be from VDL Electronics Co., Shenzhen, China

MEMS Microphones for Voice Call and Surrounding Sound

Antenna

Double Side Foam Tape

With the battery moved out the way, we can see the USB and Charging Interface connections, and Speaker Chamber

USB and Charging Interface Connectons - well marked and with test points

Speaker Chamber