Apple Watch Distance

And if it is a cellular Apple Watch and you are paying your iPhone cellular carrier to share the iPhone phone number, then you can be across the country from your iPhone, as long as you are still in the area covered by your carrier.

There is 1 restriction, in that is you want to send/receive SMS/MMS messages, the iPhone has to be on the internet, either via WiFi or cellular, but it can still be anywhere in your carriers service area.

At the moment only Apple Music streaming works.

You can transfer music you own and is stored on your iPhone over to the Apple Watch and play that. As well as podcasts and audiobooks.

Some streaming services, such as Pandora allow paying customers to download and transfer off-line playlists over to the Apple Watch.

Please read the following

https://support.apple.com/en-us/HT205547

and make sure you read the footnotes, as they contain valuable information

FYI: with my GPS only Apple Watch, I get around 40-50 feet distance between my iPhone and Apple Watch via Bluetooth (subject to walls, floors, metal objects between the 2, and other radio interference).

If the Watch is not a cellular watch, the distance would only be Bluetooth distance, generally about 30 feet. You can download music to your Apple Watch and listen with your Bluetooth headphones, so you would not need the iPhone. If you are using a GPS only Watch, you would not be able to make phone calls or receive/send messages.