[OnePlus 8T Questions & Answers] dpryor88:
I'm going to try to give you a brief, yet precise enough guide to doing the basics on a device with partitions like the 8T.

Okay, so basically, the 8T custom ROMs, when flashed, are going to consist of the following partitions:

boot

dtbo

system

system_ext

product

recovery

vendor

odm

vbmeta

vbmeta_system

So when performing fastboot/adb commands with these newer devices, you're going to start off in the normal bootloader (i use the volume +/- and power at the same time command to enter this from a powered off state).

Now there's also a mode called fastbootd mode. It's often just called fastboot, especially when booting from bootloader (fastboot reboot fastboot now takes you to this fastbootd mode). This is where you're going to do the majority of flashing/modding from.

So to start off, enter bootloader, then perform this command in terminal to do a wipe of data:

fastboot -w

Then, flash recovery partition:

fastboot flash recovery recovery.img (this is the recovery.img from the ROM zip you are attempting to flash)

Then, reboot to fastbootd:

fastboot reboot fastboot

Next, I like to make sure I'm on partition A for flashing. This is just something I've been told to do and it works so I stick with it. So do this by:

fastboot set_active a

Now you can start flashing the paritions:

fastboot flash boot boot.img

fastboot flash dtbo dtbo.img

fastboot flash system system.img

fastboot flash system_ext system_ext.img

fastboot flash product product.img

fastboot flash vendor vendor.img

fastboot flash odm odm.img

fastboot flash vbmeta vbmeta.img

fastboot flash vbmeta_system vbmeta_system.img

Then, boot up the ROM:

fastboot reboot

So that's the basic rundown.

Now for a few things to keep in mind:

Always make sure you're on the latest version of OxygenOS before doing any of this. I like to start off by unlocking bootloader, then setting up phone. Then I do an update to both partitions to the latest version of OOS (must be on both partitions. Not just on one). So I just put the latest OOS zip on my phone via USB transfer from PC, do an upgrade with that file (Settings > System > Update > Top right menu button > Select Internal Storage or whatever it says like that, then select the file (make sure the OOS zip file is in the MAIN root directory of your phone. So basically, the folder that contains the Pictures, Downloads, Music, etc. folders.

So after you do this system update, it will prompt you to reboot phone. Do this. Then, go to settings and do the same thing again. Select the OOS zip and do an internal storage update, let it do its thing, it'll ask to reboot, go ahead and do so. Now this second update you do will either start from 0% and go to 100% like the first time, or it will go from 0% directly to 99%, then 100% after a few seconds. This is okay, it just means you already had the latest OOS on one of your partitions. So anyway, go ahead and do that reboot when it tells you. Now you are unlocked and on the latest firmware and ready to flash.

Oh, and btw, to unlock bootloader, do the normal steps through developer options, then reboot to bootloader and this is the command used:

fastboot flashing unlock

That will let you confirm to unlock bootloader, etc. like I'm sure you've done on the 3T.

So anyway, after unlocking and updating to latest OOS (Open Beta or Stable usually both work fine btw), you can reboot to bootloader, then do the steps I told you above for wiping, flashing recovery, flashing partitions, booting, etc.

Oh, and to get these partition images from the custom ROM zip you choose to flash, you need to first unzip the ROM zip file, then take the payload.bin file that will result of the unzip, and use a payload_dumper script. Those are linked all over or can be found on a google search or Github. It'll take your payload.bin and turn it into these .IMG files that I list in the flashing steps. Each partition will have a file named PARTITIONNAME.img. These are all you need to flash a custom ROM over OOS.

To flash an update to a rom, do all the same stuff but don't do the

fastboot -w

and you also don't need to do the

fastboot flash recovery recovery.img

part because the recovery partition is already there from the ROM when you originally flashed.

I realize this is all a bunch of random notes and guides, and my brain kind of works in a weird way, but it's pretty much all there I think. If you need help clarifying any of this or just have any questions in general, PM me or respond back here and tag me. I'll help as much as I can or point you to someone who can help if I can't.

Hope this helps. Take care.