Question about Dual sim -I am not able to use it-

Double physical SIM cards are only for the Greater China region (excluding Taiwan), it is a hardware modification of the phone itself, giving the SIM tray and slot a different design and the SIM card reader also has contacts on the top and bottom instead of being only on the top in other models. It is also why the Chinese models have different model numbers from all other models. This exception is made to the Chinese models because China doesn't provide eSIM service, but all other countries around the world where iPhones are sold do.

The only iPhones that use dual physical nano SIMs are those made for sale in mainland China, Hong Kong and Macau.

All other iPhones sold around the world use one physical nano SIM, and a programmable eSIM.

You guys? This is a voluntary user to user forum. Apple Inc is not participating here.

And the document you linked to in your post title DOES indeed make the distinction between the models sold in China, Hong Kong and Macau and everywhere else. It is not the USA models that are different. It is the China, Hong Kong and Macau models that are different from EVERYWHERE else.

Other than those three regions that support dual physical SIMs, iPhones sold EVERYWHERE else do NOT support use of dual physical SIMs. They do support dual numbers through the use of one physical SIM and one programmable eSIM.