you can install an UbuntuTouch splshscreen on some devices as well, see here:
https://rubencarneiro.github.io/rubencarneiro.io//Ubuntu-Touch-Device-Bootlogos.markdown/

cool ! I note that flash Xiaomi Mi A2 bootlogo deletes fastboot image, fastboot mode works but instead of this image :
there is the black screen

Then I found this restored fastboot image and created my bootlogo :

The procedure is not difficult and is well explained ... perhaps it can also be used (adapted) to different devices.

Sure it has no practical effect, but I updated the splash screen on my OnePlus One, and immediately felt much cleaner!

unless its' Apple or Google, most users are not interested

To be perfectly honest, if the choice were between Apple and Google, I would choose not to have a smartphone at all. I'm absolutely serious about that.

you can install an UbuntuTouch splshscreen on some devices as well

Thanks, but that won't be necessary. I was just concerned from a purely practical standpoint that Google might still somehow be able to get my data and monitor my behavior. It would defeat the whole point of having a Linux phone.

I would point out that these start up phones, like the Pinephone, Fairphone and Vollaphone are expensive for running an operating system that is not mainstream and will only be useful in the fickle smartphone market for the enthusiast, which there is not a great deal of.

When I got my PinePhone, I was surprised how rough around the edges the OS still was. That was about two years ago. It has gotten a lot better since then, but still nowhere near as polished and functional as UT.

I am not a fan of the amount of data we give away to corporations and really impress on people who are interested (not many) the importance of UT in data security. I also wish proprietary hardware and drivers become a thing of the past, as much as it is innovation, it also holds innovation back and allows useable and useful devices to end up in landfill.

I think it's actually much worse than that, if people like Yuval Harari are to be believed. Big Tech already has the computing power and algorithms it needs to take surveillance and control to the next level. The only leverage we still have is to withhold our data. We can (still) refuse to engage with these platforms.

As you know

I actually don't know very much, so I don't want to make any assumptions. It's all still very new. I only recently began using Ubuntu as a desktop and server OS, and even more recently switched from Windows Mobile to PinePhone running Manjaro. I grew up with Windows. I have an old MCSA cert and have done extensive programming under Windows. But over the past two years or so I've realized that I need to rid myself of Big Tech in general: Google, Microsoft, Amazon, Facebook, Apple. It all has to go. But back to the matter at hand, I also installed UT on my PinePhone a few weeks ago, but of course, as you say, it's not viable as a phone for everyday use yet.

I would like to add that the Google splash screen is just that, a splash screen. It is not an indication that some piece of Android or whatever is being started.

Thanks, that's reassuring!

BTW, Android and iOS are based on Linux.

Android runs on top of a Linux kernel, this is true. But it is a misconception that iOS is also based on Linux. iOS is based on BSD, which could be considered a 'cousin' of Linux.

before installing, I needed to downgrade the Android version on the used Google Pixel 3a XL that I bought to a specific version.

The purpose of that is to bring the partitions back to the configuration that the installer expects.

However on android native devices, UT needs some parts from android to run closed source hardware for wich drivers only come with android.

So UT runs on top of some remnant parts of android, wich are, to simplify, device vendor Linux kernel and hardware drivers in it + android services, wich UT controls, that can "talk" to this hardware using those drivers.

Those parts don't communicate with google or any other entities outside ubports, if that's what bothers you.