Is it possible to use same modem firmware on different phones? Can I flash a modem file of a phone to another model? If we can't flash the modem file can you tell me what hardware or software the modem file depend so that it is different for different phone? 

You might be wondering why I want to flash modem file even if everything is working fine is that I got volte by using this method.

As login is required to view the link I will explain how I got volte. I download the firmware of yuphoria 5010a which is mostly similar to my phone's specifications. And also downloaded the stock rom of yuphoria 5010a. After that I flashed firmware, rom on my phone using fastboot. I then updated the volte patch which the phone manufacturer released for that phone and installed it and I got volte working on my device.

But the only downside was that I am only able to use volte with 32 bit rom not the 64 bit rom.

Also can you tell me how to clone or backup my entire device (including the critical file such as modem, boot,etc) so that while I would be doing lot of fiddling I can restore my device to its current state without losing anything including the volte which I obtained by flashing the modem file?

My Device : Yu Yuphoria
(5010)

ROM : MIUI 8.1(android 5.1)
No, you can't flash firmware designed to work with one android device to another one.

There are some bizzare situations where this does work because devices have identical hardware (Coolpad F1 and Micromax A310), but this is very unlikely to happen and you shouldn't try it.

As for a device backup, you can make a nandroid backup using CWM or TWRP and copy it to your computer.