Richard Cox wrote:
How do I mark a model as a favorite?

on the web browser version...

Go to "my stuff" >> Top 12

Which friends would you like to display on your profile page?
• Let me choose which friends

Richard Cox wrote:
How do I mark a model as a favorite?

Friends and favorites are not the same thing.
There are three dots at the top of the page. Move your cursor over the dots and you will have the opportunity to mark a member as a favorite. These favorites are private.