Phil's Lab at https://www.youtube.com/@PhilsLab is a very underrated channel, especially if you've ever struggled trying to wrap your head around Kalman filter techniques. Highly recommend that one.

The Signal Path isn't that obscure but there may still be some test gear nerds who haven't run across it: https://www.youtube.com/@Thesignalpath

Same is true for Marco Reps: https://www.youtube.com/@reps

FesZ Electronics at https://www.youtube.com/@FesZElectronics has some great coverage of analog/RF basics, as does W2AEW: https://www.youtube.com/@w2aew/videos

Yes, most of these folks have thousands of subs, but there's a reason for that: they're pretty awesome.