Myelin is a lipid-rich (fatty) substance formed in the central nervous system (CNS) by glial cells called oligodendrocytes, and in the peripheral nervous system (PNS) by Schwann cells. Myelin insulates nerve cell axons to increase the speed at which information (encoded as an electrical signal) travels from one nerve cell body to another (as in the CNS) or, for example, from a nerve cell body to a muscle (as in the PNS). The myelinated axon can be likened to an electrical wire (the axon) with insulating material (myelin) around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Rather, each myelin sheath insulates the axon over a single section and, in general, each axon comprises multiple long myelinated sections separated from each other by short gaps. Each myelin sheath is formed by the concentric wrapping of an oligodendrocyte or Schwann cell process around the axon.

Answer this question: the myelin sheath that covers many cns axons is formed by? Let's give stream of consciousness first:
The relevant information is: Each myelin sheath is formed by the concentric wrapping of an oligodendrocyte or Schwann cell process around the axon.
So, the final answer is the concentric wrapping of an oligodendrocyte or Schwann cell process around the axon.