Samsung IAP for Galaxy Watch only works if your Galaxy Watch is paired with your phone.
The IAP Client for watch communicates with the IAP Client for phone, which internally manages communication with supporting IAP services and the Samsung ecosystem (such as Samsung Account, Samsung Checkout, and Samsung Rewards). In other words, it acts as an intermediary between a watch app and the Samsung IAP phone system.

Samsung IAP supports credit cards by default in countries where Galaxy Store is a paid service.
Phone bill, PayPal, Samsung Pay and local payment methods (such as Shetab, Wechat, Alipay, Paytm) are available in some countries.

For more details, see the Galaxy Store Seller Portal.

Yes, non-Samsung devices paired with Galaxy Watch through the Galaxy Wearable app can use Samsung IAP.

In-app items can only be sold in countries where paid services are available.

Galaxy Store must be installed on their phone.
They must be signed in to their Samsung Account, which must have an acceptable payment method registered to it (such as a credit card).

Galaxy Store publishes the free and paid apps that offer in-app items.

To make complex apps that sell in-app items, you must use the Tizen Extension SDK.
However, if you simply want to create a Watch Face that prompts for payment after a free trial period, you can use Galaxy Watch Studio (formerly Galaxy Watch Designer).

For more details, see the Galaxy Watch Studio online tutorial

If your application is set to TEST mode, the subscription cycle is automatically renewed every 10 minutes, and the subscription is automatically canceled after 12 renewals.