Stumped on Cisco Native VLAN

I’ve watched several explanations of the native VLAN, everyone says it’s for untagged traffic.. but don’t all interfaces already have a VLAN?

So we know all ports on a cisco switch are assigned to either (A.) default VLAN, or (B.) custom VLAN. With that being said, the purpose of the native VLAN is to assign untagged traffic through the trunk port to a VLAN. My question is.. does that mean "untagged" traffic ONLY comes from the default VLAN?? Anything else would be tagged as it’s specified VLAN right?