Is interoperability a thing that Justin Sun used in connecting the Steem and Tron blockchains together or am I grasping at straws?

I'm trying to find a reference point in trying to understand side-chains and anything related to connecting blockchains with other blockchains.

Does Bitcoin, Ethereum or other cryptocurrencies use interoperability in any of the projects or networks which may be built on top of them? Maybe my questions are too generic, vague, or too silly in some ways.