How exactly does "Sharing" exercises and routines work?

You can share private exercises and routines only with a "Pro" membership.

Normally, private exercises and routines can only be seen and used by the creator of the exercise/routine. If a "Pro" member shares it with another "Pro" member, then that member will have access to it as well. In fact you can select multiple members on the site and share private exercises or entire routines to them. You can also unshare them if you need to.

This gives the creator full control of who will be able to view and use their content.

Do I need to share every exercise to someone first in order to share a routine to them with the private exercises on it?

No. If you share a routine to someone, and all the exercise on the routine are either public exercises and or private exercises you created, then the recipient will be able to load and see all exercises on it even though you did not specifically share the exercise to him/her.

It is implied that you want the recipient to use the exercise in the routine when you share the routine with your own exercises you created.

What happens if the person I shared my private exercise to saves a routine with it and then tries to "Re-share" it with someone else?

Routines saved with someone else's private exercises on it cannot be shared. This way, the private exercise is only seen and used by specific members that the creator chooses.