In the Peter Jackson movie, once the One Ring is thrown into the volcano in Mount Doom, why does the mountain and tower collapse, and the ground disappears everywhere where the orcs are standing but not where the humans are? Where are all the orcs running to, what do they do next?
The Tower collapses because its foundations was built by the power of the Ring. When the Ring was destroyed, the foundations crumbled, and the tower fell.
The mountain collapsed for much the same reason. Sauron had reshaped the volcano in the past, and those changes were being un-done.
The ground collapsing under the orcs and not the humans was a visual effect added by Peter Jackson and his crew.
The Orcs fled in fear and madness after the will of their Master was removed.
No, evil still exists in the world. One of my favorite post-Ring chapters "The Scouring of the Shire", was completely removed from the movie. It deals with the evil that men continue to do in the world.
As above, there is quite a lot of story, post-Ring, found in the novels that was removed when the movie was made. But, even in the movie, Gandalf and Bilbo leave Middle-Earth for the undying lands.
Sauron built the tower of Barad-Dur using the Ring, and when the Ring was destroyed, so were all the things Sauron had built with it.
In the books, the Orcs are not swallowed up by the earth, but many of them are killed. The rest scatter and go back to their usual habits of waylaying people, robbery, and low level criminality.
Evil is not eradicated from the world, but the singular evil that became incarnate in Sauron will be replaced by many smaller, more difficult to conquer evils.
Gandalf is immortal. He goes to the Undying Lands with Frodo and Bilbo, as well as Galadriel, Elrond, and a number of other characters.