[General Topics] roperp:
OK I've got my phone up and running Lineage OS 18.1 (https://xdaforums.com/t/gsi-11-lineageos-18-x-gsi-all-archs.4205461/). The process seemed to apply some rules from a diversity of sources, so I'll try to recount them here:

1. Enable ADB: to enable USB Debugging (ADB), through the stock (original) ROM, then, once authorized, checked driver status in my Windows 10 device manager to make sure I had no driver errors. Performed the "adb devices" check to make sure that the serial was reporting correctly. Once I did, "adb reboot bootloader" got me to the first fastboot mode.

2. Two fastboot modes: the first fastboot mode is the one with the tiny text "fastboot mode..." at the bottom of the screen. This is, as I understand it, "Fastboot" mode, not to be confused with "Fastbootd", or Dynamic Fastboot mode. (ref: https://source.android.com/setup/build/gsi) To switch to dynamic fastboot mode, the command "fastboot reboot fastboot" should get you there, with the red "fastbootd" text at the top of the screen, blocked by the camera.

3. Unlock bootloader: per all the warnings this zero's the device in irreversible ways. I tried "fastboot flashing unlock" per other websites, but that didn't work. "fastboot oem unlock" works, but I can't remember if I did this in fastbootd or fastboot mode. It didn't seem to harm anything to simply push that command through a variety of ways.

4. Flash vbmeta: At first I attempted to flash the Umidigi Bison vbmeta.img in the Fastbootd mode, since the Umidigi Bison does use dynamic partitions (ref: https://developer.android.com/codelabs/using-android-q-gsi#3), but the flash failed with message "FAILED (remote: 'No such file or directory')". On a whim, I went back to generic Fastboot mode (using the phone's keys to navigate to that menu option), and tried again. This time, the flash was successful: "fastboot flash --disable-verity --disable-verification vbmeta vbmeta.img". However, to be safe, I noticed that there were two other vbmeta files in the stock Umidigi rom (https://community.umidigi.com/forum.php?mod=viewthread&tid=19926&extra=page=1), so I flashed those as well, using some deduction: "fastboot flash --disable-verity --disable-verification vbmeta_system vbmeta_system.img" and "fastboot flash --disable-verity --disable-verification vbmeta_vendor vbmeta_vendor.img"

5. Flash system: this was the trickiest. Had to discover what dynamic partitions meant (https://source.android.com/devices/tech/ota/dynamic_partitions/implement) and how to manipulate them. First of all, it's important to be in dynamic fastboot mode to complete these things. That's fastbootd, read at the top of the screen on the Bison, blocked in part by the camera. If that isn't what's read, and instead the screen reads "fastboot mode..." in tiny letters at the bottom, issue the command "fastboot reboot fastboot" and that ought to work. Once in Fastbootd mode, I issued "fastboot erase system" which completed successfully. But don't be fooled; we're just fiddling with dynamic, "logical" partitions, so that command isn't the whole story. Instead, when I attempted to issue the command, "fastboot flash system lineage-18.1-20210317-UNOFFICIAL-treble_arm64_bvS.img" I got the error, "Resizing 'system' FAILED (remote: 'Not enough space to resize partition')". Reading up on this here: https://source.android.com/setup/build/gsi I found that I needed to make room, deleting the product logical partition (neat infographic here: https://source.android.com/devices/tech/ota/dynamic_partitions/implement). What's key, in deleting the product partition, is to heed the instructions, "The postfix _a should match the slot id of the system partition, such as system_a in this example." In the error message above, the partition is merely "system", not "system_a" or "system_b", so I issued the command: "fastboot delete-logical-partition product". After doing tihs, the command "fastboot flash system lineage-18.1-20210317-UNOFFICIAL-treble_arm64_bvS.img" worked without error.

6. Wipe. Per the instructions out there, we're instructed to wipe, so I did: "fastboot -w". It produced mixed results: "'

Erasing 'userdata' OKAY [ 2.800s]

Erase successful, but not automatically formatting.

File system type raw not supported.

Erasing 'metadata' OKAY [ 0.006s]

Erase successful, but not automatically formatting.

File system type raw not supported.

Finished. Total time: 2.817s"

7. Reboot. With fingers crossed, I rebooted. The device seemed to take longer than usual to hit the first boot screen, with it's tiny message about insecure boot because of the unlocked bootloader, and then a little longer and Lineage OS was BOOTING on my phone!

I'm sure I missed some critical details, but I'm all ears to help out.

PS: I am unclear which architecture file we're supposed to use. I used the "ab" versions of the Linage, but strictly out of peer pressure: it was the most downloaded version by far: https://sourceforge.net/projects/andyyan-gsi/files/lineage-18.x/