What is this whole process of creating A surface to B surface called in general ??

just in general.

Accepted answer

Generally it called as Close volume process in automotive industry.
You can watch this close volume video
https://youtu.be/FNhWv0Idlls


2 Other answers

surface modeling ???

In general its called 'Creating B Side Attachement.'
or Creating 'Assembly Features with consideration of Design of Assembly."