Customer Meta (Custom User Profile Fields In Backend)

Hi, what is the proper/most stable way to add custom user profile fields (meta data) that appears in the wp backend for administrators only (i.e. client cannot see these fields when he logs into his account). I saw a couple plugins that say they do this but thought maybe there is some simple code that can be added to functions.php file without having to install a plugin?

I also noticed that some customer data importers (ex. ex. “Import users from CSV with meta” and “WooCommerce Customer / Coupon / Order CSV Import Suite”) can create custom user profile fields by simply adding the field names as columns in csv file that gets uploaded. For example, if I want to add a custom field called “Additional phone1”, I can just define it in column of csv upload file as “additional_phone1” but just want to check what affect this has on system performance/db versus another method.

Topics is similar to https://www.ads-software.com/support/topic/how-to-add-extra-user-fields-for-woocommerce/ but this is for user profile data in backend rather than registration fields on frontend.

Thanks!