DocType fields best practices with Version 10?

Hi, sorry if this is a frequently asked question. I searched and didn’t see anything specific about version 10 (only previous versions that seemed to change):

I have a base development installation of ERPNext, and created a new app (It’s roughly a port of the Shopify integration to Etsy). I made some modifications to the Item doctype for the new app (for example, added Etsy Item ID) in the GUI. I see them in Item.json, but I’m not sure how to add them to my new app’s source so they’ll get added when the app is installed. I think <v10 this was handled via fixtures, but that seems to be missing from Bench and

Also, is there a way to incorporate exported form customizations (i.e.: apps/erpnext/erpnext/stock/custom/item.json) into a new app so they also get added when the app is installed?