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.
Actually ethereum based token uses Ethereum blockchain for transaction purposes and the code will be developed by their own according to their need by using Solidity as the language most of the times. But now Tron has come up with the mainnet that means they run their own blockchain and you can get the Tron explorer here https://tronscan.org/#/
If you observe carefully the public address of Tron it starts with "T" where as in Ethereum it starts "0x"
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 something completely different.
It means that transactions involving the token happen on the Ethereum blockchain. You can see TRON transactions happening on the Ethereum blockchain here: https://etherscan.io/token/Tronix