Each of the boards has an IO connected to a voltage divider on the battery to be able to read an approximation of the current voltage left in the battery. please check the Pinout card for the IO specific to your board.
Note: Check the helper library for your board for a function that lets you read the state.
On the TinyPICO Nano & NanoS3, you have to add your own voltage divider to your carrier PCB to read the battery voltage, or you can also add a I2C fuel gauge if thats your preference.