No. Most asteroids in the solar system are in the asteroid belt, but not all. The ones that may threaten to strike earth, for example, are outside the asteroid belt.
No. Asteroids are scattered throughout the solar system in various orbits, some of which are relatively stable. Some have orbits that pass near Earth. But the vast preponderance of large asteroids...
Yes, and the portal to the other galaxy is the Golden Key on the giant asteroid at X-11 Y-84 in the belt. You will need to beat all three planets (jungle, fire, ice) and have all three space knights...