What are Custom fields (variables)?
Custom fields work as a way to add specific user information to your signature templates.
If you have any information that doesn't fit in the predefined set of placeholders, you can simply assign a custom field to serve this purpose. After you do so, you can utilize them the same way as the predefined ones.
How to enable custom fields?
Go to Settings page > User page.
Scroll down to Signature fields settings and you'll see this:
2. Now modify setting of custom fields and add custom names according to your preference:
How to add content to the Custom fields?
As an administrator, you can edit these fields when adding details to your users' profiles and users will be able to access them on user page:
Is there way to connect Custom fields with G Suite?
Actually there is - it's called Advanced directory mapping and you read more about it here.