The current eSIM feature is available on the Apple Watch Series 3 and later models that run on watchOS 7 or higher. To use this feature, you will need an iPhone 6s or later with iOS 13 or a later version.
To check or download the latest software version on your iPhone, go to Settings > General > Software Update. You will also need the latest carrier settings. On your iPhone, tap Settings > General > About and follow the prompt to update to the latest carrier settings.
Please note although the new Apple Watch Series will have its own eSIM, you will still need to pair it with an iPhone.