Let's say i'd like to take an existing smart contract from the Ethereum blockchain and use it for my own project on the Tron chain. Apart from any technical issues: Is it legal to do that? Can i just grab it and use it for my own stuff without any legal implications or does it depend on some license agreement or am I always free to do it?
Source code. Source code never changes. (cit.)
Currently there's nothing that prohibits you from doing this, and on the contrary it's a really common conduct. And, I would say, it's specifically a best practice: reusing code already tested and audited by many peers it's ideally much safer then dealing with a complete brand new smart contract. And this is true not only for Ethereum, but in general for all softwares out there.
In any case, it's always up to the developer to provide a coherent and clear licensing model for its own product: if they don't provide a proper license you can consider that code free.
When you don't copy the code exactly, you should have no problems, because it is nearly impossible to proof that you really used their code. Because "In dubio pro reo", youre totally safe. When you do copy it exactly, you should have a look at the license, it should be specified. https://solidity.readthedocs.io/en/v0.6.12/layout-of-source-files.html#spdx-license-identifier If there is no license and no way to find out whos code it is, it should be ok to use it, otherwise just ask.
I would post the link as a comment, but I don't have enough reputation yet.