Just bout an iphone at Xfinity Mobile, Do I get apple tv plus for free?

Yes. See the following from About Apple TV+ - Apple Support

Any new iPhone, iPad, iPod touch, Apple TV, or Mac capable of running the latest iOS, iPadOS, tvOS, or macOS purchased after September 10, 2019, from Apple or an Apple authorized reseller, is eligible. 

For more details on offer terms and conditions, go to apple.com/promo. 

Note that the free year doesn't start until November 1 and the following has instructions on what to do then: About Apple TV+ - Apple Support