iPhones are great because since the iPhone 6s, all models have included the antenna bands to work on all networks in the United States. iPhones also continue to get software updates & support from Apple, even when they're four years old.
Pixel phones are the phones made by Google. They offer one of the best Android experiences. Pixel phones are also known for their excellent cameras and superb image quality. Google designed Pixel to work on any network in the U.S., so you're free to use your Pixel with any plan.
OnePlus's motto has always been "Never Settle." They pack their premium-budget smartphones with the latest technology and fast performance. Unfortunately, OnePlus phones are known for having limited network compatibility. They typically work on the AT&T and T-Mobile networks, with only the newer OnePlus phones supporting the Verizon network.