And you really believe that? Good for you! But even if they don't...
When they started out 12 or 13 years ago, I was pretty optimistic about MCU future, because it felt fresh and exciting - a whole cinematic universe supervised by a bunch of pretty smart and talented filmmakers. And while they still manage to make an occasional interesting oddity (the last one was probably GOTG2), everything about them feels cynically calculated to the point where it becomes annoyingly visible. Focus group or no, it feels exactly like the stuff where creatives are executives, not creatives. They have lost whatever soul they had long time ago. It's sad to see a giant like Sam Raimi given an opportunity to make a movie and then letting him to do his stuff for maybe 20 minutes of the movie's runtime. They rest is directed by robot.
Nope, writers, directors, producers and actors all collaborate together to make the best possible movie.
Sometimes a director will approach the studio with an idea, sometimes the studio approaches various directors with an idea.
As an example, Thor Ragnarok.
Marvel Studios told Taika Waititi that they wanted Hela, Odins death, the destruction of Mjilnor, Thors long hair gone and it couldn't be set on Earth. Taika worked on a treatment for the script and the studio liked it. Taika was free to do what he wanted to do as long as those elements were in The film.
Another example is Ant-Man. After Edgar Wright exited the film it was Peyton Reed, Paul Rudd and Adam McKay that collaborated on the film. They added the most important thing in the film, they framed the film around family which was completely missing from Edgar Wright's treatment. Paul Rudd also came up with the quantum realm being in the film.
Marvel Studios do not control their productions with an iron fist. Between 2010 and 2015 the Marvel committee in New York were actually controlling many things and frustrating Marvel Studios. They wouldn't allow movies like Black Widow, Black Panther or Captain Marvel be made, they wouldn't allow female villains in Iron Man 3 or Thor: The Dark World, they forced the water scene with Thor in Age of Ultron and the thing that broke the camels back is that they wouldn't let Marvel Studios have Tony and Steve fight in Civil War. At that point Feige was able to get out from under the Marvel committee in New York and secure even more freedom.