@stanwood Thank you anyway for your support! I will contact XDA people to understand why your RN7 has gps and not mine. Have a nice day (night?).

SOLVED

I read through all the GPS topics on the Telegram channel, and understood that 'test_gps' could report "GPS interface not found" even when the GPS is working!

I did new tests, left my house, waited a few minutes outside and found that the GPS was working (Pure Maps or Google Maps). And indeed, in this state, 'test_gps' keeps saying "gps interface not found"... I thought it was trustworthy, but it is not!

So everything is fine, now.

Thanks to @Josele13

SOLVED

I read through all the GPS topics on the Telegram channel, and understood that 'test_gps' could report "GPS interface not found" even when the GPS is working!

I did new tests, left my house, waited a few minutes outside and found that the GPS was working (Pure Maps or Google Maps). And indeed, in this state, 'test_gps' keeps saying "gps interface not found"... I thought it was trustworthy, but it is not!

So everything is fine, now.

Thanks to @Josele13

@josele13 Ok, thank you for the process. But it does not work on my RN7.

@josele13 Hi,

Please, could you go further with your explanation?

On your Redmi Note 7, if you restart your mobile on purpose, do you loose the GPS? I mean: after rebooting, does test_gps return 'GPS interface not found'?

And after that, you launch an app which requires the GPS, making the location indicator turn white, then you put your phone off (by a long press on the power button? or just screen off?), then put it on, and GPS starts working? (I suppose after having found satellites)
When it works, what does test_gps say?

I tried soft reboot, soft shutdown+restart, hard reboot (long press on power), simple screen off/on, but my GPS never works, and test_gps always returns "GPS interface not found".

I do not understand why it does not work on my RN7

@stanwood In Settings / About / OS Ubuntu 16.04 / Details, should the 3rd section ("Partie de l'image du peripherique" in French, could be "Vendor partition" in english) show a ref. to the lineage-16.0-20200226 vendor file?
On my RN7, it displays: 20211005-20810

@stanwood Thank you again for attempting to help me.

Yes, I unzipped the lineage-16.0-20200226-lavender-vendor.zip file, and flashed the 'vendor.img' file to the 'vendor' partition. I suppose it's not worth sending your 'vendor.img' to me, since it should be exactly the same one. Or does yours may be different?

Your question makes me doubt: the installer v0.8.8 does not ask for any vendor.img file, so after installation, one have to flash manually the vendor.img. Is this correct?

@keneda I don't think that this is the point.
When I launch 'test_gps' on my Nexus 5 with UT, from exactly the same location (in my wooden house), it displays lots of init infos, such as "device info / name = loc_api GPS module", and then starts tracking satellites, and may or may not find them.
I already tested the GPS location with my Redmi Note 7 that initially came with Android 10, and the GPS worked well, still inside my home. Since I flashed the RN7 with UT, 'test_gps' stops immediately with message "GPS interface not found :(", and therefore does not initiate any tracking step, which let me think that it's a matter of software/halium/drivers issue and not a question of location.

@josele13 I mean that 'test_gps" always returns "GPS interface not found :(" and that Pure Maps cannot find my position.

Yes, when 'position' option is activated, and I launch Pure Maps, the position indicator turns to white. I followed your instructions, turned the RN7 off and on (you don't mean 'reboot' don't you?), then waited several minutes, but Pure Maps cannot locate my position (indicator still white). And 'test_gps' still says "GPS interface not found :(".

Thank you for your help.

@stanwood Thank you anyway for your support! I will contact XDA people to understand why your RN7 has gps and not mine. Have a nice day (night?).

@stanwood Disappointed. I reverted back with stock "global" rom miui11/android9, tested the gps under Android which worked well, relaunched UBports installer v0.8.8 which does not ask for any vendor.img, then fastbooted vendor.img to the 'vendor' partition... and still no gps
But now the fingerprint security works! Before that it did not: a little sunshine in my cloudy sky.

@stanwood Thank you for the advice.
What is exactly the original official rom?
My RN7 came with Android 10, but since I bought it by a well-known used phones seller, they may have upgraded the rom when resetting the phone.
Ok, I will try restarting the whole process, with a previous global rom with Android 9, switching from firmware v12.5.1.0 to older v11.0.1.0.
Let's go!