Some UTH's come with an internal built-in thermostat, or the "package" includes it. Like these: https://www.amazon.com/Aiicioo-Under...dp/B07WC83SQZ/

Others have an integrated dimmer/rheostat. You don't want one of these: https://www.amazon.com/Spiido-MET-Ce...dp/B08L172X6D/

Or they may have nothing at all, in which case you would purchase the thermostat separately.