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Technically, the function steem.broadcast.comment(). The function is available by pulling in the steem-js library. By default it connects to steemit.com's public steemd nodes but you can easily have it connect elsewhere instead. The public nodes make it easy for developers to kickstart/develop their own projects without additional infrastructure.

Does this only allow you to build a frontend app around the steem crypto? or can you use this to create your own token/coin and web app? does this have to be built on a server or can you do it all local? what if I wanted to make it more like twitter or instagram? or pull in apis from Google for youtube or search to build something new? does it build it on top of steem? or could I rename and make my own blockchain named creem? hypothetically?

That is what the upcoming steem smt's are for: https://smt.steem.io

Do you know (or can you share) when the SMT dev tools will be available? I am very excited to dive in.