The cells themselves are pretty good - pretty much only every come across Lipo's even in "non-rechargeable" disposables. mAh basically depends on the size of the vape / the puff count, but yeah I normally find them to br between 500 and 1500 mAh (give or take).
I figured any disposable vape would have its batteries glued in. How straightforward a task is it to remove the battery? What do the batteries generally look like?
The cells are generally just "push fit", maybe with some kapton tape or foam to give them a snug fit. As for taking the vapes apart, A pair of pliers and some brute force is typically all that is needed to get them apart.
Cell shape is either cylindrical or rectangular, depends on the vapes shape really. You get the knack of remembering/guessing what the shape will be just by looking at the vape.
Came here to mention Big Clive's salvaging. Haven't tried it myself yet - the feral youth around here tend to be more whippet oriented than vaping, sadly.
My town is more a "active nightlife" town, so while I do see a far number of Whippets cartridges (though, thankfully, a lot less recently), you mainly find more discarded disposable vapes on Saturday and Sunday mornings/afternoons then you will whipped cream cartridges.
They're cheap but rechargable lithium-ion batteries, as well as a blob of electronics to make it heat an element when a button or microphone sensor activates to vape, and a lot of times, also a fancy screen with animations.
Some disposable vapes even include a USB port to recharge that battery, which is why it's certain that they're rechargable.