how to create custom virtual field

Summary:

we are currently exploring VBS and we saw that we can create a one field containing other fields through virtual fields

i found this documents however, it was not discussed how we can associate custom virtual fields to existing object

we are trying to create a custom mobile phone virtual field that includes country code, mobile area code and mobile number

however I'm stuck in the part of template as we can't find any document on how we can associate those fields in the existing one

here is how our new custom templates looks like, what we wanted to do is to associate these 3 fields to the existing mobile fields