Separate switching from power
An older thermostat may act mainly as a switch in the heat-control circuit. A smart thermostat also has electronics that need power. C, when used in a conventional low-voltage arrangement, is common power, not ground and not another heat-request function. Honeywell Home explains the R/RC supply and C return for the products covered by its power-up page. Honeywell Home power-circuit explanation.
Some smart thermostats can use certain existing control circuits for power; others need a separate supported common connection or accessory. Google's Nest guidance specifically notes that some heating-only systems require a C wire or compatible power accessory. Nest heating-only power requirements.
Do not take that example to mean every smart thermostat works without C, or that every two-wire system needs the same kit.
Ask for an equipment-specific option
Share the exact old and proposed thermostat models and the heating equipment model from existing records. Have a qualified installer identify the circuit type and whether the selected control is compatible. Avoid opening unknown wiring to investigate voltage yourself.
The installer can compare an approved accessory, appropriate cable work, or a compatible thermostat that better suits the existing system. Ask what work each option requires, what functions it supports, and how much of the installation is included.
Do not connect common to a metal cabinet, reuse a grounding conductor, or attach a separate transformer based on an online shortcut. The whole circuit needs to follow the equipment and thermostat instructions.
Check control, not just connection
After installation, ask the professional to verify stable power and heating response through the functions that apply to your equipment. A Wi-Fi connection does not prove the furnace is correctly controlled.
Your next step is a supported compatibility and power plan before installation. That can prevent a convenient new wall control from becoming a recurring no-heat problem.