Hello,

is there some rough outline, system overview diagram of sorts, that someone could point me to,
showing how Android, Linux kernel, hardware drivers and Linux distros like Ubuntu, relate to each other, esp. with this goal in mind?

Right now, I don't have a very good idea of what people who do this successfully are actually doing.
I have heard some install some sort of emulation software, which seems counterproductive as, next to being able to install regular linux software, one nice thing about running bare Linux on such a tablet would be to get more "steam" out of a cheap tablet vs. running Android apps on it.
~ 10 years back at my old workplace, we did some performance test to see what's faster by how much: Android Java app, "Native" SDK app, and running a program past Android, on the naked Linux kernel. Well, the latter was markedly faster than both former, perhaps not super surprisingly.
But I wasn't the one who did the in-depth work back then, so my ideas of how these tablets are set up, and at which point you can actually change something, are sketchy at best.
(I have done some software development on Linux SBCs and desktop, but never fiddled with tablets - TBH I never even used any Android device besides briefly at my old work years ago)

The other approach I heard about is trying to hack things such that you get the user interface of something like Ubuntu (touch?) while running the Linux kernel that came with the Android on the tablet. As there may be proprietary drivers.
Is that _the_ way then?
And how would that be done?

I guess there is no way (with sane degree of effort) to install some mainline Linux on a tablet?

In my particular case, I got a Lenovo Tab M10 HD 2nd gen (TB-X306F). I found a thread with a guide how to do this for a different model at some Ubuntu-touch site, but not for mine, so whether and how well this works at all seems to be very device specific, apparently...

Yes, it is very device specific.

I don't believe the M10 2nd is a particularly old device, so I doubt you'll find anything for it. You may want to check out postmarketos' site and hardware guides. From what I recall however, it was mostly devices several years old. A quick look myself, I could only find for the 1st gen.

If it is supported, you'll find it can run your typical DEs, as well as plasma-mobile (based on the KDE Plasma Frameworks) and phosh (a mobile shell based on GNOME tech).

I have tried using Ubuntu emulation on my tablet as a test and while it worked it was super slow and not that useable. Depending on the Tablet you can actually install Linux directly on it. For example, Kali Linux & Ubuntu both have a release specifically targeting Tablets and I have installed Debian on my Dell Tablet (which I can't find so can't tell the model number) without issues.

Try searching on the web for the Model and see if someone has already done the hard work to install Linux on it

Just info:
www.lenovo.com/linux
https://www.youtube.com/watch?v=k5Lfj51h_0M
https://firmwarefile.com/lenovo-tab-...-wifi-tb-x306f
https://forum.xda-developers.com/tags/lenovo/

For later research for the OP.