Yes. But they are black-and-white and silent, which severely limits the audience these days.

Some of these movies have circulated with more recent soundtracks, and those are off limits. In particular there’s a somewhat infamous 1984 version of “Metropolis” with music produced by Giorgio Moroder and Freddie Mercury. That won’t be in the public domain until 2079…

A streaming service could hire musicians to do a new score for public domain silent movies, they would then have the exclusive right to distribute the combined work.

In the US, yes (not willing to make a claim about IP laws in every country).

Although I'm not sure how big the market will be as it will also be completely legal to share and download them on internet for free.

Yes, yes.