The main thing about Steemit is that it could not work without all three steem tokens ( SBD , STEEMPOWER , STEEM) thanks to them, we see steemit working.
How is it possible to gain from making posts?

You can get money from the advertiser, but that is not how steemit works, what is the source of income then?

It is these three tokens.
People trust in them- they are willing to buy them because it may yield a return, but the thing is that as long as we see people trusting (buying) steem, we will have steemit rolling.
Because every upvote generates some tokens , which we can sell later on market.