What does a Bitcoin ATM actually consist of?

Over here (UK) they're dwindling. A bunch of laws and such has seen their number decline heavily in recent years but my crystal ball says that'll turn the other way in the coming 2 years.

Whilst I'd like to invest in something physical that can earn an income 24/7, I don't believe these are that something - too much maintenance and too many unknowns about both the laws and future of how people will use Bitcoin (although they will, heavily).

I'm very curious; can anybody ELI5 what a Bitcoin ATM consists of? How it works, what the components are etc. I'm quite interested in learning more about how these machines actually operate.