No such thing as a "professional" photographer, bruv. That would require a barrier to entry, a licensing body, national organization, continuing education, a code of ethics, etc.
Instead, there are three branches of photography: fine art, journalism, commercial.
Journalism is what it sounds like: photojournalism, but also war photography, documentary and nature photography, current events, scientific photography. It should at least make an attempt at verisimilitude.
Next is commercial photography, which is what most people associate with "professional" photography. It includes advertising, portraiture for hire, modeling and fashion, wedding, product, and basically any sort of contract or commissioned photography whereupon you are paid for delivery of a product and service (most architectural photography, headshots, corporate photography, etc).
Lastly is fine art, which doesn't have any forebearance or attempts at accuracy or truthfulness. Instead, it endeavors to create some new insight, a new way of seeing. Most of the time, whenever you think of a famous photographer, they were a fine artist. From Ansel Adams and Stieglitz all the way up to Todd Hido, Alec Soth and Andreas Gursky.
So, what does it take to become a commercial photographer? Not much, besides a good portfolio. But otherwise, as long as you're getting paid, you're a commercial photographer and that's about it. There's not like some gate you walk through or something like that. It's literally just being the best huckster.