Have a look here for a quick intro :
Below the video ( when viewed on YouTube ) is a list of additional links for things like the actual mirror build, the software setup, calendar setup etc.
Obviously you make it your own. I made my mirror a lot thinner ( 22mm ) so it looks like a regular mirror, not a box mirror. I put conduits into the wall for the cables and the "bulky" Pi sits 5m away in a easily accessible cupboard
I didn't do the Alexa bit in the video as I just don't have need for it. I have however added some additional modules since ( the photo is from Dec 2019 ) like Moon Phases. There are loads of modules you can add which can be found here :
https://github.com/MichMich/MagicMirror/wiki/3rd-party-modules. The screen powers off at 22h00 and on again at 06h00 when not in use