I want to type a letter from my phone

There's a beautiful Apple app called Pages. See this support guide:

Pages - Apple

and/or

Set up your document in Pages - Apple Support

Regards

Giulio

since my serial number and my imei number is no longer valid anymore in the future, how do I get a new serial number and imei number when I upgrade to 512GB on iPhone 10S? How do I ban permanently all from turning on Two Factor Authentication in the future because I don’t like Two Factor Authentication?

It's quite unfair to post a question that nothing has to do with the original poster's question.

Please open your own new topic with your questions.

Regards

Giulio

You don't. The serial number, etc. are hardcoded into the phone. You can't get a 'new one' without getting a new phone.

2 Factor authentication is now required for certain features of iCloud.