A fleet is manageable to exactly the extent that four things work: every device has a verifiable identity applied in the factory, configuration can be changed remotely per device or group, firmware can be updated safely, and each unit reports enough about itself to be triaged without a site visit.
Staged update is the part that repays attention. A firmware release that fails in the field is expensive in proportion to how many units received it, so releases go to a canary group first, are monitored against defined failure signals, and roll back automatically if those signals fire.
Provisioning is the part that repays attention earliest. Identity and credentials applied inconsistently at manufacture produce a field failure rate that looks like a hardware problem and is not, and the diagnosis takes months.