Hello,
Some people have had difficulty satisfying this prerequisite to installing Ubuntu Touch on the Pixel 3a XL...
I had my share...!
So, in case this helps anyone:

After flash-all.sh & adb sideload methods failed with errors... and TRWP wouldn't load (fastboot boot twrp.img) or install...
I persisted, flashing individual .img files - still not booting...
TWRP eventually came up! (fastboot boot twrp.img)

• I used TWRP to adb sideload the Google OTA Android 9 .zip (to slot a)
• Then I used adb sideload with stock recovery (to slot b)
• I used stock recovery adb sideload (to slot a)
• Then I used stock recovery "Factory Reset"
• Android 9 booted.
• UBPorts Installer ran perfectly.

What seems to have worked for me:

• Persistence
• Sideloading Android 9 OTA to BOTH slots a and b
• Factory Reset in stock recovery after all that

[I use a Linux PC; latest adb & fastboot;]
[latest version Google Stock Android 9 files]

Don't give up.
KJU

Hello.
Ubuntu Touch is running nicely on Pixel 3a XL bonito.
Many thanks for that.

In order to make a full backup and store it off-device, can I safely boot TWRP over a USB connection with:

• 'fastboot boot twrp.img'
• and only use the backup function
  ... then reboot UBTouch without installing TWRP?
  And, would the backup be TWRP-restorable?

I have faith that somebody knows!
Cheers,
KJU

However, 'Android Flash Tool' by Google makes the task of restoring stock factory Android to Pixels...
So... much... easier.
(It took me a while to find it, that's all.)

https://flash.android.com/welcome

Access is by opening that website in Chrome or Edge or another of several compatible browsers.

When it completes, it offers to lock the bootloader, but that can be skipped.
KJU

However, 'Android Flash Tool' by Google makes the task of restoring stock factory Android to Pixels...
So... much... easier.
(It took me a while to find it, that's all.)

https://flash.android.com/welcome

Access is by opening that website in Chrome or Edge or another of several compatible browsers.

When it completes, it offers to lock the bootloader, but that can be skipped.
KJU

@arubislander @Moem
Creative comments.
Of course, my concern was not to rename, but to understand. Still, 'Fresh Install' would be perfectly clear, and true to the language of Linux.
KJU

@KJU
My question has been asked and answered in these forums before, I find.
If 'Wipe' & 'Factory Reset' would give me a fresh Ubuntu Touch install (often a great friend in Linux!) I see I'll need to use fastboot to flash UT away.
Solved.

'Factory Reset' in other contexts implies 'original' or 'as-sold' - thus my uncertainty, and others' before me. The 'Yes' button is, of course, one way to find out...

Cheers,
Kurt

Hello,
I am impressed with Ubuntu Touch, but I'll return this little bonito to LineageOS soon.

Before I do anything:

• What does UBPorts Recovery's 'Wipe / Factory Reset' leave me with?
  (The Recovery screen notes 'Halium 9' & 'slot a')

My hopeful guess is stock Android 9 ( slot b?)
In that case TWRP ought to boot, and let me sideload stock Android 12 OTA images, required for LineageOS.

My reasons:

• battery drain seems quicker in UT
• the UT virtual keyboard and many other features would suit me better on a small tablet.
  UT Tweak is great, though!

Cheers,
Kurt

@KJU
Hello,
TWRP, it seems, doesn't boot temporarily with my Ubuntu Touch bonito device in fastboot mode.

• 'fastboot boot twrp.img' gives 'no CMD or OS...' error.
  KJU

@AppLee I've had another go with the usbotg-dongled mouse.
On my device, it's not doing everything I'd like, or I'm missing the way... Or is it my mouse?

• I can see open applications, by swiping from right side, but cannot position the pointer to swipe any up & away.
• The mouse doesn't open the application drawer with a swipe from the left [aha! just solved this one - push left edge and wait]
• In Morph browser, how can open tabs be closed by mouse?
• Scrolling is very slow.
  Cheers,
  KJU

Hello.
Ubuntu Touch is running nicely on Pixel 3a XL bonito.
Many thanks for that.

In order to make a full backup and store it off-device, can I safely boot TWRP over a USB connection with:

• 'fastboot boot twrp.img'
• and only use the backup function
  ... then reboot UBTouch without installing TWRP?
  And, would the backup be TWRP-restorable?

I have faith that somebody knows!
Cheers,
KJU

@AppLee Thanks for the tip. I like the UBTouch gestures, (which didn't seem to respond to the mouse) but the virtual keyboard hasn't impressed me, so I'd forego the mouse.
Cheers,
KJU

@MrT10001 Now mine shows up automatically too! However did I miss that?
Thanks indeed.
KJU

@libreboot
Hello,
I have UBTouch 20.04 OTA4 on Pixel 3a XL bonito.

• Microsoft Wedge Bluetooth keyboard connected straightaway. Very welcome, especially in Morph Browser!
• New Logitech Pebble Bluetooth mouse showed 'connected' but no cursor appeared on the Pixel's display.
• Old Logitech Pebble failed the same way on Bluetooth.
• Old Logitech Pebble connected with its little dongle on a USB OTG adaptor. Cursor appeared and functioned.
  (But it doesn't replace the basic UBTouch gestures...)
  Cheers,
  KJU