Etherium based tokens When we say certain crypto currency is based over etherium, for instance TRON, does it mean they have used etherium code from github and modified as per their use case? Or is it something completely different?
It is completely different. 
Ethereum offers a standardized way to create new tokens on its blockchain called the ERC-20 protocol. ERC-20 is not a piece of code, software, or technology. Rather, it is guidelines that facilitate the integration of various currencies.
Tron is an ethereum based token that means it is powered by Ethereum blockchain. It will use resources of the ETH blockchain Tron is not a blockchain started from scratch.