Not specific to 40K, but Goobertown Hobbies is one of my favourite channels. It’s a mini painting channel, always positive and beginner friendly, with a lot of useful advice regarding painting techniques.
For Hobby History, I recommend Arbitor Ian. He's an old grognard like me who has been collecting for decades, and explains some of the interesting changes and old lore of the setting (stuff from as far back as when Space Marines all had "born to kill" type graffiti all over their armour). He also does a "Tale of four gamers" type thing with some other hobbyists, and it feels a bit like those old White Dwarf articles come to life. Lots of nostalgia, but also new lore too.
I'm gonna try and mention ones that I haven't seen other people mention yet ...
Rogue Hobbies is newer and really good, in my opinion. She's a former GW employee who went solo recently and focuses on painting.
Poorhammer as a YouTube channel or a podcast is great, they are fun to listen to and get some basic insights here and there.
Auspex Tactics is a fucking work horse for getting news, reveals, and in-depth analysis. Dude uploads like crazy and is almost always the first I see covering news. His 10e coverage was GOAT-level for the entire lead up to Leviathan's release
PlayonTabletop for battle reports, Poorhammer for just generic 40k entertainment, Luetin and Weshammer for lore, Auspex Tactics for rules updates and news, and Ninjon and Eons of battle have become recent favorites for the hobby aspect along with Midwinter Minis.
Pile all that on with the rest of my interests and my algorithm is having a seizure...
The Exploring Series on YouTube is moving back into WH40K lore. Been watching him since SCP lore series and since he's doing Warhammer i figured what the heck I'll check that out too.