Welcome Arun_1995,
Thank you for using Apple Support Communities. Based on your post, you have some concerns with your iPhone 12 Pro and possible rust at the bottom. We'll do our best to assist.
It appears there is damage or rust, as you described, at the bottom of your iPhone 12 Pro. For this issue, we'd recommend bringing in the iPhone to a local Apple service center for an inspection.
This can get you prepared for the visit: Get your iPhone, iPad, or iPod touch ready for service
Our link here will help to locate the service center. Select your region and search nearby here: Find Locations
Due to the current conditions around the world, some stores may have limited hours or appointment times. They also may require specific things to visit them (Such as, scheduling an appointment, wearing a mask, calling to check-in, etc.
Wishing you the best, take care!