Dealing with any copyright issues is, of course, your own problem but assuming you have taken care of such issues:
The Python libraries:
youtube-dl - Can download videos from a number of sources
MoviePy - Can easily extract sections of videos and combine them into a sequence or montage. Note that MoviePy installs and uses FFMPEG as a back-end
All of the ...
While the biggest part certainly depends on the video (and especially the video quality/resolution), the official YT app also consumes bandwidth for ads and such. So maybe you want to take a look at Newpipe, which is a free/libre open-source alternative to that. You won't find such clients on Google Play (as they do not meet the Play Store guidelines) – but ...
Another easy way to accomplish this task, which will also work for things like podcasts, is to use FakeStandby.
FakeStandby is available via F-Droid: https://f-droid.org/packages/android.jonas.fakestandby/
It is open-source software and is gratis.
I have personally used it, and found it to work very well.
FakeStandby will turn off your Android device's ...