c type 20w original charger which is coming from iphone 12 pro max

The iPhone 12 pro max does not come with any charger. If you want a 20 watt charger you must buy it separately. However, if you have one you can use it to charge an iPhone 7. The charger will “know” it is an iPhone 7, and will only supply the voltage and current that the iPhone 7 requires, so it will not charge the iPhone 7 any faster than a 12 watt iPad charger would (which is a little faster than the 5 watt “cube” that the iPhone 7 came with). It won’t hurt the iPhone 7 to use a 20 watt charger, however.

What the repair shop told you is absolute nonsense unless it wasn’t an Apple or Apple-approved 20W charger. Likewise, if you weren’t using an Apple cable or an Apple-approved Made for iPhone cable that might have caused failure of the phone→Why Counterfeit Lightning Cables Kill iPhones - Motherboard