Headphone amplifier and audio interface question

[Cruxfold]

I plan on getting a pair of Sennheiser 660s and an XLR microphone once I get some extra cash, but I'm starting to get confused about the setup. From what I understand, I will need an audio interface for the microphone and a headphone amp for the headphones (to get the best experience). But I don't know if it's best to connect the interface and the amp, which one to plug the headphones in if they do connect, and how best to connect the amp and interface for a desktop computer setup. I know most amps feature RCA I/O connection, and most interfaces feature the RCA output connection, so I'd likely need a RCA to 3.5mm adapter, but do I plug both in the computer, or do I setup a train with them somehow?

What is the most optimal way to setup both the mic and my headphones with the amp and interface to my desktop computer?