How to add a trusted device , my iphone 12 Pro Max to my ID

See the following Apple articles:

About the 'Trust This Computer' alert on your iPhone, iPad, or iPod touch

About the 'Trust This Computer' alert on your iPhone, iPad, or iPod touch - Apple Support

Authorize or deauthorize your computer for iTunes Store purchases

Authorize or deauthorize your computer for iTunes Store purchases - Apple Support 

Get a verification code and sign in with two-factor authentication

Get a verification code and sign in with two-factor authentication - Apple Support

need to add my PC to the trusted device list, can't find directions

See the 'Add a device to the list' section in the article below.

Apple ID device list -Check to see where you‘re signed in.   *