Unbound is a dns resolver and don't need any upstream dns.
see https://docs.opnsense.org/manual/unbound.html, you can even find someting about query forwarding and dns over tls there.
System > Settings > General is a little different, see https://docs.opnsense.org/manual/settingsmenu.html#general
If you chose mullvad instead of unbound this is totally fine, but as you found out it will bypass unbound depending on your settings.
To use unbound you don't need any dns server in general settings, just uncheck `Allow DNS server list to be overridden by DHCP/PPP on WAN` and uncheck `Do not use the local DNS service as a nameserver for this system`, thats it basically.
Or if you prefer check `Do not use the local DNS service as a nameserver for this system` and add 127.0.0.1 to servers explicitely.
Query forwarding or DoT should then be configured under services > unbound itself IMO.
see https://docs.opnsense.org/manual/unbound.html, you can even find someting about query forwarding and dns over tls there.
System > Settings > General is a little different, see https://docs.opnsense.org/manual/settingsmenu.html#general
If you chose mullvad instead of unbound this is totally fine, but as you found out it will bypass unbound depending on your settings.
To use unbound you don't need any dns server in general settings, just uncheck `Allow DNS server list to be overridden by DHCP/PPP on WAN` and uncheck `Do not use the local DNS service as a nameserver for this system`, thats it basically.
Or if you prefer check `Do not use the local DNS service as a nameserver for this system` and add 127.0.0.1 to servers explicitely.
Query forwarding or DoT should then be configured under services > unbound itself IMO.