There are many variations of secondary carpet backing. Actionback is a poly woven fiber for example. This is one of the most common found in US.The two most popular ways is glued on with adhesive or stretched in, they cannot shrink.
Softbac is a spun backing made specifically to avoid scratching hardwood flooring. Jute is plant based material that can brown easily when cleaned improperly. Unitar has an extra thick coat of adhesive instead of a secondary backing. This way the fibers are stationary ad very difficult to pull out. Unfortunately this can only be glued on.
Squares can be cut with cushions with the main benefit being the adhesive releases if a square becomes worn. Needle punch is the only construction that does not use traditional yarn. This material is spun one layer on top of another and then needle “punched” into a backing. Most walk off mats are made using this method.