[General Questions and Answers] mcc111:
Hello, I have moderate experience as an Android developer but I don't know much about custom ROMs.

Last weekend my cell phone died and for a temporary replacement I walked into an iffy local repair store and asked them for the cheapest Android phone they had. They gave me a preowned Z3352CA which I think is a "ZTE Blade A3 L". It is running Android 9 Build number Z3352CAV1.0.B03 ("Android Go"). It's pretty nice. However I have a sense buying a preowned Android is probably about as safe as sticking your hand in a garbage disposal. I did a factory reset first thing, but I would like to be reasonably certain the thing the factory reset installed was a legitimate ZTE OS with no extra presents from the previous owner like spyware*.

I asked a question in the ZTE Blade forum but got no responses, which is unsurprising for a question about a old and obscure budget phone. Trying again in here-- I understand people probably don't know about ZTE/ZTE Blade phones specifically, so I would like to ask some general questions about how to use fastboot or what you'd expect with a normal Android 9 ("go") device.

Specifically I found an iffy article describing bootloader unlock procedures, with this screenshot:

View attachment 5421571

"Device tampered" caught my eye. Some discussions I see online about fastboot reference this "device tampered" flag. I am interested in this. If I had to guess, I'd guess this flag is supposed to go true if the device is unlocked and stay true even if the device is re-locked. To be clear, right now I *don't want* the bootloader unlocked. What I want is to be sure nobody before me has ever unlocked the bootloader.

When I saw this I attempted to put the phone into Fastboot Mode. I can't tell if this worked or not. I started up with the "volume up" and power buttons held, which dropped me into "Recovery Mode". It said "Android Recovery" at the top and offered a menu of options, one was "Reboot to Bootloader". I selected this and was sent to a black screen with "=> FASTBOOT mode..." in the bottom left corner in a terminal font. Nothing happens at this screen. The up and down volume buttons, and the power button, do nothing (I can boot the phone normally by holding the power button down 3 seconds). I can't tell if this is fastboot mode or not. If I run fastboot.exe devices on my Windows 10 machine, it is not able to see or connect to the phone in "=> FASTBOOT mode..." mode. (I have the normal Windows Android drivers installed for running adb but have not installed the ZTE specific drivers.)

I dropped the phone into Developer Mode and looked in Developer Options. I saw an "OEM unlocking" checkbox, which is unchecked. I thought: Maybe this will let me talk to fastboot. But if I tap it, it says:

Allow OEM unlocking?​

Warning: Device protection features will not work on this device while this setting is turned on.​

I tapped "cancel" because at this point it occurred to me this switch, by itself, might qualify as "tampering".

(Other things I tried: I ran the app "SafetyNet Test" from Google Play and it said I passed. I enabled USB debugging and was able to connect to the device with adb. I tried "adb.exe reboot bootloader" but it just took me back to the "=> FASTBOOT mode..." screen. I looked on the Android Developer website for documentation of "fastboot oem device-info" but couldn't find anything.)

TLDR, what I am trying to figure out:

Is fastboot oem device-info documented anywhere? Is it an Android thing or a OEM specific thing? If its meaning is not fully standard, are there conventions?

What should I expect the "Device tampered" key in fastboot oem device-info to mean?

What effect should I expect the "Allow OEM unlocking" checkbox in Developer Options to have (either on fastboot, or on the "Device tampered" key?

Can either fastboot.exe or adb.exe tell me whether the device has ever been bootloader unlocked?

What will checking "Allow OEM unlocking" in Developer Options do? Will it allow me to talk to Fastboot? Will it result in the phone being flagged as "tampered'?

Generally is there anything I can do, with the tools available to me (ADB, and fastboot if I can make it work) to ensure a used phone has not had durable malware added and I am running a vendor-stock OS?

* I am also interested in any info as to the probability ZTE itself or the ZTE default ROM is spyware-ing me.