[OnePlus 7T Questions & Answers] s7101:
Hello guys,

I recently bought a used OnePlus 7T, with the goal in mind, to try to install Ubuntu Touch on it. There were some tries to do this in the past, there is posts on the forums here where a 7 Pro was successfully running UT.

Now while I am a software developer, I am a total beginner regarding 1) UT porting 2) Android in general. I have never used an Android device, and I am unfamiliar with the OS and all related terminology. The latter seems to give me some headaches at the moment, so I would like to ask you guys for some starting help.

When I received the phone it was running OOS 11, and I downgraded to OOS 10 as described here after I unlocked the boot loader and also installed TWRP (I think this was not needed yet, but well ...). And I think I didnt pay enough attention to it, since now it does not seem to be the EU version anymore; the about page tells me: "10.3.1.HD65AA", which I think means global? Does it have any impact on the functionality of the phone? Can I switch back to EU version? If so, how? Should I pick a EU-zip from here and flash it again?

Now since I was trying to downgrade even further to android 9 (in order to begin with the Ubuntu Touch related stuff), I tried to find a ZIP containing the OOS 9 downgrade, but I could find none. The closest which I found was this. However when I place the downloaded ZIP in the root folder ("OnePlus 7T" is the root folder? E.g. the one where I see subfolders like ".OTA", "SysQS", "TWRP"?), the system update will not pick it up in the local upgrade menu. It just doesn't find it. I already renamend it from .jar to .zip, but still it does not work. This makes me believe I downloaded the wrong ZIP.

Long story short, can someone help me to achieve the following:

1) Switch back to EU version

2) Downgrade to OOS 9

And I have some general questions:

1) TWRP is a tool which is installed on the recovery partition of the phone, and it can be booted into to install various things. It would for example allow to install a different Android version, as alternative to system -> software update -> gears -> local. Is that a correct summary? For which step would I actually need TWRP during the a) the downgrade process from 11 -> 9 and b) porting to UT?

2) Does the phone need to be rooted during the downgrade process, or when porting to UT?

3) What are the two slots which are shown in TWRP? Are those 2 partitions on the internal storage?

4) How many partitions (in the regular PC sense of partitions of a hard disk) does the phone have? 2? One for recovery, and one for the operating system? In the UBPorts docs I read that the vendor blobs reside on a separate partition since Android 9, so there must be at least 3 partitions on the phone? How do I make sure that I don't accidentially write e.g. on the vendor blobs partition? I guess that `fastboot flash recovery twrp.img` means that the *recovery* partition should be flashed, right?

I think thats it for now. Sorry for the wall of text, but its just so much information for a beginner, and its a bit overwhelming. But I want to get into it bit by bit, and maybe in the end contribute to the device porting, in case I manage to do it.