Well, Physical custody is a custody that is held by the parent or person with whom the child lives whereas Legal custody is the custody which is an ability to make decisions that would be affecting the child in terms of health care, education, etc.
Thus, when there is either type of custody it is also possible for it to be joint.
An example of joint physical custody would be like where a child would divide his/her time between each parent’s home. Whereas , Joint legal custody would occur when both of the parents would be sharing decision making and also other duties.
Therefore Joint custody will be involving both the parents to share their responsibility and power with respect to their children; also it is quite likely that it may also involve joint legal custody and joint physical custody.

Hope this answers your query, walsh.