How to find a field name/column name of a field in database where I have only the label of that field?
You can derive the database table name by the host entity and the field name. The name schema is ENTITYTYPE__FIELDNAME

Views don't directly show the field name, but you can look at the entity's "Manage field" page to get the field name by label. (e.g. for nodes /admin/structure/types/manage/BUNDLENAME/fields)

Examples:

node__body
node__field_image
users__field_whatsapp
taxonomy_term__field_text


Exception:
Some special fields (like title or created) are stored as a column on the base table of an entity. Base tables follow the schema ENTITYTYPE_field_data, e.g.

node_field_data
users_field_data
taxonomy_term_field_data