@Hadron bro I mean to ask that the twrp recovery image should be installed on mobile device permanently or it can work by loading virtually through fastboot recovery twrprecoveey.img . I have already took the backup from twrp backup menu after loading it through fastboot recovery twrprecovery.img and backup is done successfully I think hopefully the bootloader is not locked but still as backup is reading from rom and copying writing to a image file hopefully it don't need write permission but the recovery menu is a process of writing flashing to the rom and need writing permission privillages .. so for unlocking bootloader which command should be u
Fastboot OEM unlock
Fastboot flashing unlock
So the main question is can I recover write back to ROM back from the backup images taken by twrp backup menu through virtually loading (fastboot recovery twrprecvery.img) not installing or I would have to install the recovery image on to device itself permanentally
I don't know a "fastboot recovery" command myself. There is "fastboot flash recovery <recovery image>" to flash the recovery image to the recovery partition, i.e. permanently install it, and "fastboot boot recovery <recovery image>" to boot the phone once into the recovery without installing it.
Anyway if you've managed to take the backup by just booting into recovery without installing it permanently there's no reason you can't restore it the same way. In fact not all phones allow you to boot into a recovery that hasn't been installed - I even once owned a phone where you could do that on the original firmware but a later update removed the option - but if that works then it should make no difference whether the recovery is permanently installed or not (other than needing a computer if it is not installed).
Bootloader unlocking is separate from recovery. If you can fastboot boot into a custom recovery then as long as you are using the recovery I don't think it will matter whether the bootloader is unlocked or not. But if you want to install a custom recovery via fastboot, or use fastboot to flash anything other than the manufacturer's signed firmware images, then you do need to unlock the bootloader.
Whether you use "fastboot oem unlock" or "fastboot flashing unlock" depends on what device you have. "fastboot flashing unlock" is newer, so if this device is on 5.1 my guess would be "oem unlock". Either way beware, as that will probably factory reset the phone, so make sure you have backups first (and files on the device do not count as backups).
