A:AnswerSE doesn't have an Always-On feature, ECG, and Oxygen level sensor. Other than these 3 features, it is mainly the same device (unless you want stainless steel model)
A:AnswerBuilt-in Apple Health of Fitness apps do not have this functionality. But you can install third-party apps that allow it. Like MyFitnessPal, for example
A:AnswerI've had the SE watch for 6 weeks & yes you can do both with my 7plus. I didn't add it to my carrier & turns out I didn't need to (unnecessary added expense). I can receive texts & calls even when I'm walking in the woods; not hunting for my phone. I especially like dismissing a call immediately when it's interrupting something; generally speak to my watch to reply to messages & am impressed with it's transcribing ability.