Hi philgib,
=> Can I get 5V from the Seeeduino with a 3.7V rechargeable battery connected to the 3.7V port ?
It is not possible.
=> I am getting lost now at how I can recharge that 3.7 battery without the expansion board
Once XIAO is connected to USB, the 3.7V battery is automatically charged.
=> Would you suggest a simple way to connect the 3.7v battery, the Seeeduino and the 5V motor without frying the Seeeduino ?
XIAO cannot drive a motor directly. A 5V-300mA or higher motor driver(FET or transistor) and a power supply are required.