During a recent tire change the tech did not reinstall the round black hubs. Do these hubs screw on or are they just kind of friction fit over the round tubular shapes screwed in from back of the wheel rim? I don't really see any screw threads on interior of bosses on the hubs.
Thanks.

You can glue those nuts back on to your center cap and then they screw on from the back. Need to take the wheel off.