01 / Definition
A digital twin of a building is not a 3D model. A model is a static description of intent. A twin is that description joined to live or periodic data about how the building is actually behaving, which is a categorically different thing to build and a much harder thing to keep.
Most working examples are far narrower than the marketing implies: one asset register, joined to one sensor feed, answering one operational question that somebody has to answer repeatedly anyway.
02 / The four layers
static
Geometry and object data, inherited from design and construction models.
asset register
The list of things that must be maintained, with identifiers that survive the handover from construction into operation.
telemetry
Sensor and building-management data, arriving continuously and needing to be tied to the correct asset.
context
Occupancy, work orders, tariffs and anything else that makes the telemetry interpretable rather than merely present.
03 / Where it fails
The joining problem is harder than the modelling problem. A twin only functions if the identifier on a sensor matches the identifier on the asset in the model, and those two identifiers are typically created by different organisations, years apart, with no shared convention.
Twins built without a defined operational question become expensive visualisations. And data that is not maintained decays quickly: a twin reflecting the building as designed rather than as it now stands is worse than no twin at all, because people trust it.
Aside
The parts that never went remote
For all the dashboards, commissioning and handover verification still happen with people standing in the building. Someone physically checks that the asset tag on the plant matches the record, because no amount of modelling resolves a mislabelled valve.
Which is why project travel stubbornly persists. Teams landing in South Florida for a walkthrough tend to pre-book the ground leg: an airport-to-airport hop between Miami and Fort Lauderdale when the site sits in Broward, or a run down to the PortMiami terminals for marine and terminal assets. Where the visit is a client-facing review rather than a survey, that usually upgrades to an executive car booked for the day.