There are a ton of things you can do with a keyboard natively on Android. I would imagine these use that plus a bit of software to emulate "real" movements, like the flicking down to scroll

Sure with like teamviewer or similar app. I'd guess they're running the bot on a computer which is remote controlling those phones

Yes, you can control the phone programmatically if you unlock it and put it into dev mode.

If it's just a browser you can connect to chrome devtools via web socket and send commands