The code is all available and under a libre license I might add. From

Not the proprietary drivers that vendors may build into their versions of Android.

From a kernel perspective you can’t just port a newer kernel. That’s not how Android works. It

Tell my why?

It has been done on some devices but it is very hard and usually very impossible for one reason or another.

The kernel is highly modified, yes. Porting is a ton of work, which is exactly what I said. Thats the reason why there is no support for newer kernels most of the time.

you get the firmware from the upstream manufacturer such as Qualcomm.

Yes and if they stop sending it, you dont get security patches here anymore. Cant sign it, cant patch it. What I said.

as the modem runs its own software and is a total black box.

Absolutely. Luckily Airplane mode is pretty good.

I don’t understand your motivations

People here tell a lot of wrong things. Like that LineageOS could support devices fully after they are dropped by their vendor. Or that running some old version of Android is fine, while they just focus on UI elements.