Each coil is hand-tied, usually 8 ways, then individually wrapped in cloth. The springs should be close enough to prevent sagging, yet not so close that they rub against each other. The filling is next. The most luxurious is down. It is also the most expensive.
Polyurethane is the most popular filling used today.
The dacron wrapping will help keep the fabric in place. If the cushions have zippers, you do not have to trust anyone on this. Just unzip and see for yourself. Finally, you have the outside covering. It can be vinyl, leather or fabric.