Hello, Altavia!
Thank you for your question.
I can see two scenarios here:
1 - Let's assume you have a many2one field on the model Z to the model X: in that case you can set up a related field to get the field you created (on Z) on X.
2 - What are the 'Existing Fields'? those are fields that are on the current model and not in the view, invisible or not.
I hope I helped!
And to know more about Studio and other Odoo applications, you can schedule a demo with an Odoo Expert: https://www.odoo.com/r/w2t :)
Cheers,
Odoo eLearning Team.