How do I set this device a trusted device
To set thiis device as a trusted device
iPhone 6s, iOS 14
From the article below.
Trusted devices
A trusted device is an iPhone, iPad, or iPod touch with iOS 9 or later, or Mac with OS X El Capitan or later that you've already signed in to using two-factor authentication. It’s a device we know is yours and that can be used to verify your identity by displaying a verification code from Apple when you sign in on a different device or browser. An Apple Watch with watchOS 6 or later can receive verification codes when you sign in with your Apple ID, but cannot act as a trusted device for password resets.
Two-factor authentication for Apple ID - Apple Support