Most commonly RC battery packs are referred to as "1S, 2S, 3S," etc. indicating how many single cells there are in the pack wired in series. Those are also individually rechargeable via a low-current charging cable.
Yep. I'm very familiar with these from quadcopters.
The charging cable is usually called a "balance plug", because it distributes power equally to all cells instead of shoving it all through all of them in series.
Wired in series, mAh won't change, wired in parallel, it's additive.