How to hang a heavy mirror with rotating support hinges?
50lbs is definitely something drywall or plaster can handle, but absolutely go with using the studs. Just regular wood screws or decks screws are fine, 2" to 2 1/2" long maximum to avoid any unknown electrical or plumbing. You can also double the screws on top of each other for more security.

Using the hinges or rings as they are will keep the mirror more vertical &amp; tighter to the wall, but you have to very carefully level the screws based on the hinges or rings being under some amount load or weight. They may be out of level with each other or one may also be bent higher than the other.

Using the hinges or rings as anchors for a hanging wire, which is their intended &amp; normal use, will bring the top of the mirror out from the wall &amp; pitch the viewing angle down a little. But you only need 1-pair of screws side by side in one stud &amp; leveling is done by centering the wire on the screws.

The short wall studs could indicate concrete, brick or cinder block construction halfway up the wall. The whole garage wall should read this way &amp; was typically done to withstand slight car impacts. If it's just a 2 to 4' area then it could be the remnants of a fireplace or chimney.