Apple Watch questions

The key here is that the iPhone must always be in Bluetooth range or on the same home WiFi for the Apple Watch to use the iPhone cellular radio.

If you want to leave your iPhone at home and still have all those services, then you need cellular model. The cellular model, when the cellular is activated, shares the iPhone's phone number, so the iPhone carrier has to support the Apple Watch. For this service, the iPhone carrier charges a monthly fee. In the U.S. the fee is $10 per month added to the iPhone's cellular bill.

Yes if you have the GPS only version of the watch then it uses the sim card on your phone to make calls and send texts.

You have to have your phone with you to be able to do this.