Comment by mdorazio

I’d love to know, too. If it was me I’d use a small piezo plate on the side of the balloon to do it the same way hot air balloons do. Heat the gas in the balloon = go up, reverse the polarity to cool it = go down. That would be pretty energy inefficient, though, so hopefully their secret method is better.

Loon used pumps and an interior air ballast like blimps do. So clearly there are a few ways.