The Seeeduino Wiki claims that it can run on 3.3V/5V. It doesn't specify if it can step down higher voltages. I am looking to power the XIAO using a battery. It usually uses a USB-C cable which can provide 5V. Will I need to step the voltage down, or does it behave like a normal Arduino Nano and automatically step down the voltage?
If you scroll down on that page you link to you can see the power circuit. It uses an XC6206 voltage regulator which has a maximum input voltage of 7V and a dropout voltage that could be as high as 680mV.
So you can feed between about 4V and 7V in to the VIN pad and it will step it down to 3.3V. Anything below 4V and you will go "out of regulation" though the system may still run down to lower voltages. If you go above 7V you will blow it up.