The United States bishops’ conference (as well as a few other bishops’ conferences elsewhere in the world later on) applied for, and received, an “indult” (or temporary, but renewable, approval) to allow United States Catholics to receive Communion in the hand
if they choose.
In the Universal Church, the norm has always been, and remains, Communion on the tongue.
I am not sure the progression of this indult, I am too young to have dealt with it myself

but it seems like Communion in the hand started after Vatican II
without approval in the United States… then the US bishops applied for approval from the Vatican and received it.
Remember, though, it is
not the norm, only an indult, and that means that Rome may rescind it’s approval for Communion in the hand whenever the indult comes up for review and renewal…
Also, remember that if you travel elsewhere in the world, it is probably best to assume that you should always receive Communion on the tongue, since many places do not receive in the hand (and hopefully no more places ever will!!)
+veritas+