What does it take to open an NFL franchise?

First you will need to be able to show you have the resources to finance a team.

Yes, the New York Giants and the New York Jets both play out of the same stadium in New Jersey.

Yes the NFL has a monopoly, they decide who and how many teams they will add, then accept bids from competing markets and select the ones they think have the best chance of succeeding.

There have been many attempts in the past, most recently the USFL. None of them has been successful since the AFL, which merged with the NFL to form the current league.

Millions and millions of dollars to spend.

Permission to form a team.

If you want to start your own league, you will need millions and millions of dollars more for marketing and to sustain initial losses.