Modifications in “Custom Field”, “Property Setter” and “Custom Script” or for that matter any DocType, can be exported into fixtures by adding them to hooks.py of your app.
fixtures = ["Custom Field", "Custom Script", "Property Setter"]
then export fixtures:
bench --site mysite export-fixtures
More : https://frappe.github.io/frappe/user/en/guides/app-development/how-to-create-custom-fields-during-app-installation.html