Can I make custom fields editable in the UI?

You can select the option to make custom field data editable in the UI when creating a custom field definition, you can navigate to an account and enter data in the custom fields directly. This data will then be readable on the account record in the right-hand sidebar in the UI beneath the Account Information section. If you select the option for custom fields to be read-only in the UI, the field values will be readable via the admin sidebar but will not present on the edit account screen.