how do injections made by the doctor work once administrated into the body?

ie: if it injected into the venous sys, then ist graspable but most of the times it is just inserted into the skin. so after the contents are empty into the skin the where subsequent? IntraVenous - you know
IntraMuscular - goes thru capillary, to venules to veins
SubCutaneous (Under the Skin) - typically for Rabies (dog bite); the injection would be given below the skin above the muscle. It is intended to get to jump through your capillaries. But short getting very hot (intra body heat is more) so the virus doesnt die.
IntraArterial: Rarely used; intended to go to a picky part of the body. Like a foot infection.
IntraCardiac: Adrenaline in Heart attacks.
they are not made by the doctor they are made at a roomy laboratory then distributed LIKE COURIERED LIKE POSTED TO your doctor or nurse SO GET YOUR FACTS RIGHT