What file is the list of domains for my cert stored in

Certbot uses the list of subject names in the most recent certificate as the list of subject names for a renewal:

Normally if you want to change this list you should force a renewal with a new list of subject names, like

certbot --cert-name example.com -d one.example.com -d two.example.com -d three.example.com -d example.com -d www.example.com --force-renewal

If this succeeds, then you have an new certificate that covers exactly those names rather than whatever names were covered before.

I believe that this is currently the only way to do it because I believe the domains setting is explicitly ignored in the configuration file, due to a common mistake where people were putting this setting in their cli.ini files.