Conceptually: physical port (e.g. ether1) is a port. When you add L3 configuration (e.g. IP address) to it, it becomes interface. If you want to work with tagged frames, coming in and leaving out of, you have to create appropriate VLAN ports, anchored to physical port. Adding IP addresses to those VLAN ports makes them VLAN interfaces (VLAN ports can be used as bridge ports when creating per-VLAN bridge in a pre-6.42 fashion).
Likewise: bridge (named e.g. bridge1) comes with CPU-facing port, named bridge1. If you add IP address to it, it becomes interface named bridge1 (and IP stack works with this interface, not with bridge member ports. I.e. in-interface is then bridge1 or out-interface is bridge1 … not some member port). So the same as with individual physical ports, if you want to work with VLANs, you have to create some VLAN ports and anchor them to port bridge1.
When doing VLANs with bridge, it’s necessary to configure the virtual switch … so you configure VLAN membership (and PVID) for all member ports, including CPU-facing port named bridge1.
So as you can see, port bridge1 is treated the same as other explicitly configured bridge ports (ether ports, SFP ports, wifi ports) except for implicit creation. Naming around ROS is slightly inconsistent (most ports are found under /interface … ethernet or wireless or …), but never the less they are ports prior to receiving L3 configuration.