How would a dentist fill in a small cavity? Can he just laser away instead of drill?

Yes, possibly. Laser dentistry is becoming more common and some can remove a cavity instead of using a drill. Sometimes you don't even need anesthetic (shots). The cavity is then filled (restored) with whatever material you and your dentist decide is best. If you are really interested in laser dentistry i would search out a dentist that has a hard tissue laser.
Depends . The location and size of the cavity may determine how and if the cavity is filled. Dentists have used a number of techniques in treating small cavities, from sealants, and a number of different restorative materials. In the past, the old drill and fill was the primary technique used. With the advent of newer technology, some offices are using lasers to remove the decay with less discomfort.