How durable? Apparently, very durable. I've never read a post where someone complained of a digital crown failure. I've had two Watches, it's never been a problem or concern.
If you want to get some idea of how it's built, check this article about Apple's patent application, which includes some drawings. It's from 2015, so there may have been some changes since then, and the patent filing describes alternate approaches to some features, so it's not a
definitive description of exactly how it works.
https://www.patentlyapple.com/paten...gy-behind-the-apple-watchs-digital-crown.html
For those who love mechanical watches, this is something to love - it has gears and bearings; in spirit very much like a traditional watch. Also note that there are no mechanical switch contacts; it uses magnets on the shaft, which actuate solid-state Hall-effect switches - those things don't wear out.