I wanted to know if it was possible to create a system where we would make users pay to highlight their publications (like on instagram for example).
No need to be as advanced as Instagram, just a banal highlighting for example, over a chosen duration.
Yes, that would be easy enough. In the simplest form, all you really need is an attribute in the User Profiles table that indicates whether or not that user is a paid subscriber. If they are, then give their content special attention.
Exactly what @Darren_Murphy said. You could do something like this:
In the user publications table, relate back to the user profile table and do a lookup of the user’s subscription status. If they’re an active member, then adjust the sorting of the publications so their content is first.