On that screenshot - lan port 1 is tagged with vid 3, lan 2-4 are untagged, you can combine whatever you want.
A port cant be untagged member of vlan - it's wether a member of vlan and tagged or untagged completely there's no 3rd option. What you have seen with luci switch before where you can set cpu and port tagged independently - is somewhat excessive configuration, or misconfiguration if you call it.
If you want to tag a port you add .vid to it i.e. lan1.3 or 'wan.8' etc. you can add as many vids to a single port as you want and bridge em together, route or send to the moon (not really).
Yes, exactly, and they all are bridged together - can communicate with each other
All the packets that are destined to lan port 1 in that configuration are tagged with vid 3, that's pretty simple
edit: forgot to mention - after you fill custom interface field with custom interface you it apply and it's added to the list above it