A two part video that's IMO an amazing explanation of how a valve gear works to control the flow of steam into/out of the piston to make things move. (Specifically the Walschaerts valve gear.)
Part 1: https://youtu.be/dNP-TL_R4Ys
Part 2: https://youtu.be/O6rlfXCsoPk
In terms of youtube channels, I basically just watch Hyce (there's a lot of good channels I'm sure, but there just aren't enough hours in the day, lol.)
He works at a Railroad Museum and does videos on the steam locomotives which can range from "Lets put a gopro on the underside of the train and see how the wheels navigate a turn." to "lets see if we can make and cook a pizza in the 1,500°F firebox." Also plays train related games if you're into let's plays. And has an ongoing project of recreating an old locomotive in CAD to see if it'd be feasible to build from scratch.
Link to his "Railroad 101" playlist: https://www.youtube.com/playlist?list=PLPnUi7cftYHxDVFE_jx7ZAYxyUgLcw1yi
(But IMO his other stuff is worth checking out as well.)