Hello kishenraj288,
It would be impossible for anybody here to speculate on what may have caused what you are seeing.
Be sure you are following the recommendations found at Cleaning your iPhone - Apple Support when cleaning your iPhone:
These models feature glass with advanced textured matte finishes either on the back glass or around the camera area. With normal use, the textured glass on your iPhone might show signs of material transfer from objects that come in contact with your iPhone, such as denim or items in your pocket. Material transfer might resemble a scratch, but can be removed in most cases.
Clean your iPhone immediately if it comes in contact with anything that might cause stains or other damage — for example, dirt or sand, ink, makeup, soap, detergent, acids or acidic foods, or lotions. Follow these guidelines for cleaning:
• Unplug all cables and turn off your iPhone.
• Use a soft, slightly damp, lint-free cloth — for example, a lens cloth.
• Avoid getting moisture in openings.
• Don’t use cleaning products or compressed air.
Your iPhone has a fingerprint-resistant oleophobic — oil repellent — coating. Cleaning products and abrasive materials will diminish the coating and might scratch your iPhone.
If you would like to have the iPhone examined by a Service Technician, you can make an appointment at an Apple Authorized Service Provider using the Apple Support app on any iOS or iPadOS device.
Have a good day.