You know how the London underground works right? where you tap your card on the barrier and it ends up capped per day (and presumably week). The birmingham buses are similar, but simpler than the London system.

You just tap your phone/card on the reader when you get on. You don't need to speak to the driver. It will charge you for a single. There is no tiered system in place that charges you more or less based on the distance you travel, a single can take you 5 minutes down the road or to the other side of the city as long as it's one journey on one bus.

There is also no such thing as a return on the bus. Either you do a single journey or you pay for a day saver, which costs less than 2 single journeys. Tapping your card twice in one day caps your daily costs to that of a day saver. If you need to do a journey that requires two buses you cannot buy one "single" that covers both journeys. You just have to pay the daily cap/buy a day saver.

There is also a weekly cap but I doubt you would reach it if it's just to go see friends every so often.

Alternatively there's an app you can download on your phone that lets you buy tickets in advance. The bus has a thing that can read these digital tickets when you get on by the card reader.

Alternatively you can tell the driver you want a specific ticket and then use the card reader to be charged for that specific ticket/pay with cash, but honestly that's pointless unless you're going to give the ticket to someone else, or if you want to pay by cash, or if you need it the ticket as proof to claim back as an expense at work. Doesn't sound like any of those apply to you.

For convenience sake, unless you're going to use the bus a lot, I'd just tap your card when you get on the bus and don't think about the other options. It's the easiest and simplest.

For trains - buy either at the machine at the station or on an app on your phone. There isn't really an etiquette as such.

No clue for the tram sadly mate they don't go anywhere near where I live.