Yes would be the general answer. Prefab garages do come complete with everything needed for construction. This is for the basic setup. Add-ons and specific options would need to be considered when pricing.
Starting with the cement flooring it takes about two days, and from there it can be two to three more days depending on what design you chose for your garage.
"It really depends on the size of the portable garage. You need at least two people for the smaller ones. As they get larger, you will need to get more people to help."
If you mean making the components of prefrabricated garages, then they may be made by hand using methods such as woodwork and casting concrete panels. The erection of a prefab garage can also be done...