The vast majority of Passenger tires and for the most part Light Truck tires have polyester body ply material and steel belts. Most Truck/Bus tires will have steel body and belts.
In the picture above we see the two bias (angled) body ply plus the two angled belt ply in the bias-belted construction from an early Goodrich tire.
The actual number of ply has decreased over the years as technology improved and stronger cords were developed so fewer actual layers of material was needed to contain the air pressure that is needed to carry the load.
Also the switch from bias construction to radial allowed the decrease from 2, 4, 6, 8 or another even number etc body ply to 1 or 2 or 3 actual ply. This graphic is a basic representation of Bias, Bias Belted and radial constructions.
Passenger tire might say "Sidewall 1 ply polyester, Tread 2 ply steel + 1 ply polyester"
Light truck might say the same as passenger but some larger LT tires might have two or even three body ply so they might say "Sidewall 2 ply polyester, Tread 3 ply steel + 2 ply polyester"
Truck/Bus tires will probably say "Sidewall 1 ply steel, Tread 4 ply steel"
Note Any reinforcement material that goes through the center of the tread is counted so the body ply material is counted in the tread material listing.
Here is an example of a heavy duty traction design truck tire with one steel body ply and 4 steel belt ply.
To help you understand the relationship here is a simple comparison of Ply Rating and Load range
Load Range Ply Rating Load Range Ply Rating
A 2 B 4
C 6 D 8
E 10 F 12
G 14 H 16