Nest, ecobee and Honeywell's T9 are built for 24 V wiring. A fan-coil thermostat on district cooling is often mains-powered, and ducted and VRF units take their maker's own controller. How to tell which you have, and what fits each.

Battery doorbells and plug-in cameras are usually yours to fit; a smart lock or anything drilled in is a modification that needs written consent. The line is whether the device is reversible.