ONLY if your cellular service provider/plan has set it up that way, though. If tethering is treated like a part of the unlimited data, then no UI switch is needed.The switch controls how much data apps on your DEVICE can use.
If you're tethering, whatever you have tethered to your phone will see it as any other WiFi connection, and use as much data as it normally would.
Because the carrier has details about the data stream, they know what’s being tethered and what the device is consuming. They even know which APP is consuming in order to not set that against your data allowance (like AT&T providing data free HBOMax). As a result, they can alter the speed and quantity specifically of what’s being tethered (AT&T limits tethering speed and quantity separate from the phone’s native data plan).