And you notice there is no technical support about flashing your old tablets or phones from the manufacturere to download and firmware. or download section. Because they don't want people to be flashing and old phone or tablet. You neeed both custom recovery plus 'custom' rom for that operating system to work. and that .zip files is from manufacturer. most people dont have the tech ability to make custom roms or custom android for their device and with so many models and very little demand it's not worth making it.you see droid kits which only works on samsung phones or tablets and they have downloads to reflash it is BRICKED.

The problem is you need the exact customt TWRP recovery AND exact install .zip for the model number. There is no unversal twrp file or universal android, or utbuntu. Each manufacturer, model, useds a 'custom twrp file. and unique identifier for android 'custom' rom even for android itself.

if you install wrong file not for the particular device, it won't work and you need to customized or find the exact model rom known as custom rom. even if the phone is running android. you can only flash the manfucturere rom for that device. or c ustomized it yourself

oem unlocking just means you can FLASH it. most devices now have oem locked and you cannot flash anything on it. improperly flashing wrong rom can brick the device and if there is no custom rom for download, there is nothing you can do. you have make one for yourself which needs the manufacturers drivers etc.