Nest and Ecobee can work with many common 24V heating and cooling systems, including some heat pumps

https://holdeniibj353.lumenforgex.com/posts/why-a-thermostat-problem-may-not-be-the-thermostat-at-all-2

Nest and Ecobee can work with many common 24V heating and cooling systems, including some heat pumps, though certain homes may need a C-wire or add-on power accessory.