The material most four-wheeler soft shackles are made from is high-modulus polyethylene rope, commonly known as Dyneema.    This rope type is generally of an elght- or 12-strand construction and can be produced in varying widths from six millimetresto 48mm in thickness.
Most soft shackles used in 4X4 recovery scenarlos are 12mm in thickness and constructed from 12-strand Dyneema SK75 or SK78.    Both are roughly the same, withthe latter offering slightly better resistance to stretching-out(and thus shape recovery) under sustained tenslon.
As far as constructlon goes, a soft shackle couldn't be simpler.    A soft shackle is made up of a 'strop' with a stopperat one end (knot - usually a diamond knot) and a splicedend at the other.    Some shackles are often wrapped in athinner Dyneema to give improved abraslon resistance