How to hang this heavy mirror and what is this hanging method called

This is a good video that goes over the different types of hardware to use to hang different weights of items on drywall with no studs:

https://www.youtube.com/watch?v=2SSJaO3gVPE&w=560&h=315%5D

I think these are excellent: https://smile.amazon.com/Hangman-Z-Hanger-Mirror-Picture-Aluminum/dp/B002NEFF7Y/ref=pd_sbs_60_3?_encoding=UTF8&pd_rd_i=B002NEFF7Y&pd_rd_r=MMF5XV4MJR27E16WP4Y5&pd_rd_w=w48io&pd_rd_wg=HUxUD&psc=1&refRID=MMF5XV4MJR27E16WP4Y5

Those are known as keyhole slots. Hooks on screw heads and slips down the slot. You would use self drilling drywall anchors into the wall, screw the screws into the anchor and hang the mirror. Use some strips of masking tape from the back of the mirror lined up with the slot hanger to the front of the mirror over the top and side. Mark the location of the slot hanger on the front of the tape. Hold the mirror on the wall making sure it is level. Make some light pencil tick marks on the wall from your location marks. Set the mirror down. Use a straight edge or by eye and draw a light line till the top and side tick marks make a plus sign. The center of the plus sign is where you drill the anchor into the wall. Screw the screws into the anchors leaving a gap for the slot hanger. Erase your lines. Hang the mirror. Remove the tape. Those anchors will hold up to 75 pounds.

Put a piece of tape on the back simply mark the holes then remove tape an place on desired wall. Use your marked spots on tape to nail then hang it up it will be perfect!!!