The Cellular Apple Watch shares the iPhone's phone number. This requires the iPhone cellular carrier to support the Apple Watch. The iPhone carrier charges a monthly fee for this service. In the U.S. the fee is $10 per month added to your iPhone bill.
iPhone Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:
https://www.apple.com/watch/cellular
As to whether Peloton supports the Apple Watch, you will have to check with Peloton.
Not all streaming services support the Apple Watch.