@xmesaj2 let me clarify some of these things for you
p.s don’t get scared by this huge wall of text, just take your time to read through it
4.9 kernel or (currently) known as the BSP kernel, this is the board support package, or the version of linux the linux kernel which is provided by the board/chip manufacturer (Amlogic or Rockchip) and contains the most necessary or basic software packages needed for the SoC to Run, you will find that almost every accessory “just plain works” when using a linux Image with this kernel
Mainline linux or Upstream kernel, is the latest and greatest of what current software provides, it is a sort of Unstable or Beta kernel as not everything will Run in this kernel, but those that do Run will be quite optimised.
the mainline kernel takes development from made at the moment from various gropus communities around the world and is managed by maintainers and also Linus Torvalds (founder and creator of linux kernel) himself, every patch that is is accepted and sent upstream must go through him.
whenever a Linux kernel is released for us here, it also is released with its respective version of uboot,
das u-boot (the uboot) (or) Universal bootloader is a free and open source bootloader for many embedded device (or unanimously known as SBCs- single board computers)
many devices run uboot as their bootloader of choice as its free and open source and fits right in with the GNU ecosystem.
more can be read about uboot from wiki pages and their own website.
Android is the one of the most widely used mobile operating systems, as one would know, millions of devices run android as their primary operating system, its most commonly found on mobile phones.
Android TV is a port of Android which is optimized and made for TVs, this version of android is for Android TVs and Tv boxes or anything that is used for video playback, streaming purposes, and general media consumption.
hope that solves you questions
This is something that requires a bit more practice in my opinion, even after doing this process for many times now I still miss the mark and boot into recovery sometimes
basically if you hold on the power button for longer than intended when doing the keys upgrade mode, you’ll end up in android recovery (depicted by the green android “droid”),
but you can follow my step above which refers to seeing the screen, and what logo appears on the screen, check the booting process, just remember to undershoot the button press removal (releasing just it before khadas logo comes on) and you will surely get into Upgrade mode
I’ve explained it quite carefully I hope, any other questions you can ping me