How do two-way Bitcoin ATMs work?

Could you elaborate on the operational mechanism of two-way Bitcoin ATMs? I'm curious to understand how these machines facilitate both the purchase and sale of Bitcoin in a seamless manner. Specifically, I'm interested in knowing the steps involved in both the buying and selling process, as well as any security measures or verification processes that are in place to ensure the safety and integrity of transactions. Additionally, I'd like to know if these ATMs are widely accessible and how users can locate them for their convenience.