For impromptu and for strangers you run across there aren't any rules as you can't expect them to know them. I'm not so aware of an elbow flick as I am when someone points as
Carbonfiberboy mentioned. Equal times pulling or pulling till you need a break, it's sort of how well matched your abilities are that determines which.
Certainly nothing beats verbal communication. So if staying behind I'll let the rider know I'm behind them and if I'm going to be within a bike length or so I tell them "on your wheel". Once you are in earshot then you can work out the details. Sometimes if noisy background, then get side by side.
If you do come up on anyone, I feel you should always announce your presence whether just staying behind or drafting. If the speed difference is such that you are going to be passing very soon anyhow, then just say "passing left" or whatever at least far enough back so they have time to react to you. Some do stupid things when they hear you and others it apparently scares the begeezes out of. Walkers, cyclists and joggers. I had a dog walker jump out of his skin and trip over his dog just yesterday when I shouted out "passing left".
And riding with others brings up the mask issue. You are definitely safer with a mask when following even 15 or 20 feet behind a group. I've worn cloth masks and find the pleated ones seem to work better as they don't suck into your mouth and nose giving you just a small section of cloth to breath through. The pleats seem to hold enough spacing so you have more material for the air to flow through.
The ascot type things that go all the way around are just too hot. At least the ones I've tried.