Quinn (Blondihacks) on youtube is a model engineer machinist and has, IMO, the most outsider-watchable content on building boilers and steam engines like this.
Highly recommend this primer titled "The Model Engineering Learning Curve" https://www.youtube.com/watch?v=Ps_BQJEMnGA
She's been building a model locomotive for years now, there are over 100 videos: https://www.youtube.com/watch?list=PLY67-4BrEae-xhrvZX33gPk4...
One more: a little engine I'd never encountered before called a wobbler is, apparently, the hello world of machining model engines: https://www.youtube.com/watch?v=HrHDrIms7-0
Blondihacks is amazing. Beware though, you may be buying a lathe and a milling machine soon after you start watching :).
It's a great hobby. Manual machining is totally different from writing code, but you get into a similar state of flow.
See also Wreck2Restored, a channel that documents the ongoing restoration of a 2-foot gauge steam locomotive. Crossheads and sliding valves featured!
I'm only ~5 minutes into that first video and every single thing she has said so far rings so stupid true that its almost painful hearing it said so bluntly. Making things takes a lot of time, requires lots of fundamental skills, is expensive to do yourself, generally isn't going to be better than something other people are selling, and should be fun the entire time you're doing it.
This should be the comment that people read on this post. Watch these videos.