There are several Foxconn and Wistron factories assembling iphones in India. Soon the iphone 12 will also be assembled here. A third company has been licenced to assemble them recently.
Same as Japanese car makers assembling cars in US for sales in US. Because the Indian market is so large, it makes perfect sense to assemble iPhones there to get around to Indian tariffs. Same iPho0ne which costs $1100 in US has a price tag of $1700 in India with the added import taxes.