[XCSme]

But how do you do that without also having a long-lived key or access token to those services?

[noAnswer]

The long-lived credentials life inside a stripped down machine. Cron/lego/Ansible handles the renewal. The machines on the edge can't renew their keys themselves.

[XCSme]

Oh, this makes sense, so instead of "the app is rotating its keys" is more like "the keys in our app are being rotated by an external service".