It takes two CR123s. But it will burn through them quickly. It’s easy to swap from the side though. There are several after market battery extension housings you can add to it. There’s also a usb-c plug on side that allows external battery supply and to be used but it can be cumbersome and detach/ lose power during handling or recoil of the rifle. The image is top notch though.
Yes, an external battery pack can be attached via buttstock, mlok, etc. There are several companies that provide this. It connects via USB to the battery pack and Type C to the thermal scope.
There's a bunch of external battery packs you can buy. Essentially any charge bank you can get for your phone. I personally have the sightmark external battery that mounts onto a picatinny and love it.