How do Bitcoin ATMs work?

Could you please elaborate on the functioning of Bitcoin ATMs? I'm particularly interested in understanding the process from start to finish, including how users interact with the machine, the steps involved in purchasing or selling Bitcoin, and any security measures taken to ensure transactions are secure. Additionally, I'd like to know if there are any limitations or restrictions when using Bitcoin ATMs, such as transaction limits or geographical availability. A clear and concise explanation would be greatly appreciated.