I'm a bit clueless when it comes to mobile devices. And I was ignoring their security for too long.

I have 2 Android phones that has obsolete OS'es on them one with Android 6.0 another with 8.?.? both of them has many severe vulnerabilities including. Both devices does not have a slot for memory card, therefore
complicating reinstallation.

Since search engines are toxic on this topic. I was wondering maybe someone could suggest ways and tools to do this using Linux or OpenBSD ?

P.S. I'm asking this, because toxicity reachable vai search engines confused me too much. for it's unclear whether those tools universal or they're a just for specific phone models as well as major of those tools seems to be *.exe malware blobs for windows.

If relevant, phones are "UMI TOUCHX", "Ulefone S1 Pro".

Phones are not like PCs - you can't just grab a download and install it.

There are multiple steps, some (most) not supported by the phone manufacturer, any of which may turn your phone into a brick (totally useless). And somebody has to be interested enough to port the necessary changes to make you models work. I found an article for the Ulefone here, to give you an idea of what's involved. I couldn't find anything for the other phone.

Be warned this is not trivial and has a large chance of making your phone useless - I've trashed two phones; neither of which I cared about.

Edit: - this might be a better link; just another option.

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I see, I've seen those URLs by myself, but they're less than complete. That's what I tought about phones being treacherous, they are intended to be thrown out after sheeples pays the last payment for them in order to just take a new one, and this device telling me that it's up to date, when the last security update was 2018-something looking and critical vulnerabilities it has, without any official suggestions to upgrade it to the later version, seems pretty fucked up..

And these devices as I came with workarounds for this can only be connected to my own network, separated from other devices and at best to my own VPN-intranet, to make use of them without too much risks, like using any public wireless APs is basically a suicide for such devices.

I wonder what their "service guarantee/insurance" providers/sellers could say about this. I mean in my case at least I'm a bit interested to phuck around with them, but a regular smartphone user might not be aware of this for many years, running a compromised device.

This just makes it interesting to run some metasploits exploits at such devices that I own, just to be sure.

P.S.

I also must add that both devices turned out to be able to have micro SD cards, via basically same slots as for sim cards. WHich also turned out not to be too relevant.

The largest community of phone tinkerers is XDA-Developers. You can find ROMs for more phones there than anywhere else. For phones, the only way to change the OS is through flashing a new ROM into it from a computer using the ADB suite. That runs on Windows, Linux, or MacOS. It's fiddly, and requires exactly correct syntax like anything dealing with computers, and it also requires having a compatible ROM. If it doesn't exist on XDA-Developers it probably doesn't exist at all.

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I tend to agree with v32itas general thoughts & feelings about this; unfortunately the rant does not help one bit to solve the problem at hand.
You were given at least one link specific to one of your devices; dismissing them with "but they're less than complete" only 1.5 hours after they were posted does not seem right. Of course it's not going to be easy.

I try to avoid disappointment by never buying new, never spending more than, say, 100 credits, and always doing research before purchasing.

Install ADB tool to your computer
Connect your device to your computer with a cable
Copy apk file you want to install to the folder where your ADB tool is
Start your console Terminal (on Linux) or Command Prompt (on Windows)
From your console you have to go to the folder in #3 - To do this use cd command.

You sure you want to go with it, and if you don't have enough expertise you may end with two dead phones with you. And android 8 can still work for some time. You can start the experimentations with the android 6.

https://lineageos.org/
https://download.lineageos.org/

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NOT TRUE - they are non-Intel PCs, they don't commonly run X11 as a front end, it's more "SDL"ish (asian special chips) multimedia driven, more sensors than PCs usually have installed and functioning

however: the part about bricking your phone is true. only do what your manufacturer suggests. don't download anything "un-official"

Now - there are roms you can DL and use (obivously compatibility is a question). But I can't answer you on your actual question. You'd want to store the rom "on the phone", except your flashing the phone.

On the phone the flashing "does not destroy user data" meaning "NOT ALLOWED". So, that would be where you'd want a flash rom: but that might not be accessible during flashing. And what about fallbacks if it fails: does your method have sufficient fallbacks to prevent against "bricking the phone"?

good idea I think you better stick with official releases for now