Hi @Lee_G. Yes, basic questions but good ones and I had the same questions not so long ago. All great info from @fiddle_wood and I agree wholeheartedly.
I might add that it is very helpful to learn and memorize the chord progression for each tune to play backup successfully and Ben puts the chords in the tabs thankfully.
Once you have the progression down, you can change from vamping to rolling to licks to whatever on the fly. And if you can learn the chords in terms of the Nashville Number System, all the better, because then you’ll be able to play backup on a tune in any key.
In real life jams, someone suggests a tune, someone else may ask “what key?”, a key is agreed on based on what’s common or how a certain player learned the tune or who’s singing and their range and then players rush to get their capos placed just right (mostly the banjo and guitar players) and it seems it’s always the banjo player that needs the most time because the 5th string has to be dealt with as well and then some fine tuning is usually required after capos go on or come off.
Whoever suggested the song tends to be the boss or leader for that song so while you’re playing, you make eye contact with the boss now and then. If you have not made any prior agreements or they are not familiar with your skills and abilities or whether you even know this song, they’ll give you an inquisitive look, meaning hey, do have a break and are you ready? If so, you nod and if not, shake your head. If yes, then be ready for the nod or a bony finger pointing at you or they might just say “Lee, A part” or whatever.
And be careful that you don’t go too far. The leader may already have others lined up for breaks so they start their break while you’re still playing yours and bloody chaos ensues, oh the humanity…
Now, I’m no expert, I haven’t played in jams yet because I’m shy and just chicken you know what. And, I’m still learning backup and the NNS. I do attend jams regularly though and these are just my observations.