This is more of a confirmation, but if I am right, I think that the jannisaries were elite infantry units made up by the devshirme.
Devshirme is the system (itself) of recruiting Christman boys into the Ottoman army. Jannisaries ARE the elite infantry units you’re referring to.
One creates the other. The Devshirme system took in often Christian children taken in battle or tribute and raised them Muslim and educated theme. These children often then became Janissary as they served no other political or familial obligation.
Devshirme is the process used by the Ottomans of recruiting soldiers/bureaucrats/laborers from among the children of the conquered Christian children.
Janissaries / janizaries were the elite fighting force that protected the sultan. Because the (ostensibly) weren’t Muslim, they wouldn’t have any other allegiance to anyone or any family beyond the sultan.