Notice how long the credits are in any major motion picture. That's a lot of names — hundreds of people are involved in making a big movie. All of those people are doing this as their job, and have to be paid. Some of them are paid quite highly.
If a movie is based on a book, comic, TV show, video game, etc. then there's also licensing for the story rights; which can be millions right there. On top of that, movie-making equipment is expensive (and typically rented); modern CGI special effects take a lot of computer power, which costs money too; major movies also have big advertising budgets.
Look at the average scene in an MCU movie and see that 90% of it is usually CGI. Extrapolate all those effects across a 120 minute film and add in the price of the top name actors they usually have, and I'm surprised it's only 350 million.
I can only speak from an audio perspective, but for most american films you dont use anything you record on set. That means thousands of hours recording each and every sound, placing it into the film, and mixing it to sound good.
So factoring in the gear cost, work hours, recording studio, etc it can easily reach a few mil on sound alone.
Edit: To answer the second question, bigger production indie films have become a lot more popular post 2020. The smaller staff sizes mean each area (usually) has more creative control; resulting in better films for farrr cheaper.
They also can't afford expensive (and not always better) production techniques, like not using on-set sound.