How are elbow fractures usually treated?

It varies. All fractures are different. Non displaced fractures are usuallly treated with immobilization in a sling, splint or cast. Fractures that are displaced or involve the joint surface frequently need surgical treatment or at least pinning.
Depends. Some elbow fractures such as fractures of the radial head can sometimes not require any treatment at all other than therapy. Other fractures may require surgey and this depends on the location and severity of the fracture. An x-ray and evaluation by an orthopedic surgeon is needed to make the diagnosis and determine the correct treatment.