This is about hvac units: how is a transformer connected to get power and then give power to the thermostat

And where is the transformer excactly - lets say on a residential a/c unit with gas furnace

Transformers,, officially called step down transformers,can / are located in various places at the choice of the Mfg. usually somewhere that the least amount of wire is needed. In a gas furnace there is 120 voltes fed into one side and the transformer steps the power down to 24 volts,the 24 volts is what feeds power to the tstat and the tstat distributes the 24 volts to control what ever the stat is set to be needed

Hi if there is no power measured across the 24 volt termanals, with a volt meter, using the lead connectors does it matter where to place the test leads? On the termanls?