How to set your own avatar and information on Apple's mobile phone?

After it is selected, you can see a "rectangle with upward arrow" sign in the lower left corner of the photo, and click it.

After clicking, some options will pop up at the bottom, and select "Assign to Contacts";

Select the contact to add a picture in the contact list, then move the zoom to adjust the picture size, and click "Select";

In this way, you can display the selected photo as the avatar of the contact, then improve the personal information such as the phone number of the contact, and then click "Update".