Search the HBAL227 in ADS-B exchange and zoom in to the selected balloon. You will see a total of 6 of these pilotable balloons in a cluster. HBAL227 flew there singly to join up with the rest.

They don't drift. They fly. And HBAL227 flew around the US before joining other balloons, now in Mexico.

Read the cover story about these balloons at Loon.com. Dig and you'll find the method of navigation, that Loon is a subsidiary of Alphabet and more.

And why are they on the Mexican side of the border? If they are to give communication services to Mexico, why not launch the balloons inside Mexico instead of flying them around the US over areas of medical research or military facilities first before going across the US/Mexico border? At 60,000' they fly with great stealth.