Does Ubuntu Touch actually replace Android or does it somehow run on top of it?

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@messayisto

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.