Transformers are to transfer energy or signals in an electrically isolated way. While power transformers ought to have a low loss core material, signal transformation may work good enough with cheaper material. Mostly ferrite is being used and there are at least two windings. More secondary windings enable to trigger more switching transistors like in a bridge circuit.
In order to meet electrical strength insulation layers or multiple isolated wires is being used.
A special variation is flyback chokes, a combination of transformer and storage choke. In this case metal or iron powder cores may be adequate, as well. However, ferrite is mostly used.
Advantages of toriods vs. E-core types and its relatives are
tape wound core (BB)
amorphous / nanocristalline tape wound cores
metal powder (HF)