the short answer
Fitness influencers make money through brand deals and sponsorships, affiliate marketing, selling their own products like ebooks and programs, paid memberships, and - the highest-margin and most durable option - selling online coaching to their audience. The smart play is to stack a few low-friction streams early, then turn the trust you have built into paying coaching clients.
This article is general information for creators and coaches, not legal, financial, or medical advice - certification, tax, and disclosure rules vary by country and change over time, so verify the specifics for your situation.