welcome to tangled! There are several reasons to make a post, one is just to express yourself or post something that you enjoy that maybe someone else will too, another is to make money. In order to make money on a post, you are correct, people have to engage. It costs 10,000 millix to post, so to make money you need a combination of reactions, comments, or tips adding up to more than 10,000.
Another good way is to grow your team, by subscribing to people, reacting to their posts, and posting quality content people will subscribe to you. Also, inviting your friends and family makes your team bigger.