Answer: they learn mighty fast!
We are given an insight into this process on YouTube by the great video-maker digitalalexa.
In less than forty seconds at the start of this video, a banjo player picks a tune - Who Walks In When I Walk Out, gives the others an outline of the chord sequence ('G minor... B.... Breaks on Bb...' etc.) and how they will tackle it; and off they go. No problem. Enjoy it
by clicking here. See them even take a chorus at 4 mins. 10 secs.!
By the way, if you would like to play Who Walks In yourself, here it is: