Do custom records take precedence over any, some, or all default values ?
If you try to create custom records with the same name and type as generated records, they’re combined. If you try to add a second A record, you end up with two, and clients will pick one of the two at random.
To remove non-editable DNS records, disable the service that is creating them. For instance, to remove non-editable A records for www, set the domain to DNS-only.