How do ISPs monitor the data usage?
Your data traverses their network and their equipment. Most if not all commercial and carrier-grade equipment has monitoring capabilities built-in.
Is the technique used by them Standard or does it differs from one ISP to another?
A good starting point for looking into this is searching the web for "CALEA", and even the now inactive project "OpenCALEA."
does using Tor or VPN prevents it from doing this?
Tor encrypts data and bounces around your traffic among several computers running Tor before it gives it to an exit node. The exit node then decrypts your traffic and forwards it on to the destination.
Encrypting data prevents intermediate hosts such as your ISP from being able to see the contents of what you send, but not the time, amount of data, or source/destination IP of what you send.
Tor's IP obfuscation will likely prevent your ISP from knowing the eventual final destination IP of the data - if traffic bounces between ISPs, which depends on how many people are running Tor and where. The more people that are running Tor in diverse locations, the better this works.
Your ISP can detect you are using Tor if you use the normally available public Tor nodes because a public list of these needs to be available by design for you to use Tor.
Is there a way you can bypass this?
Don't send data via your ISP.