I don't think `aeroway=terminal` + `building:part=` is a good idea. "1 `building=` to be divided into domestic and international terminals" seems to be the operational perspective. They should be different indoor features in the same `aeroway=terminal` + `building:=`. If they just use 'Domestic SECTION' and 'International SECTION' as names, but everything else as the same, do you still think they should be separate `aeroway=terminal`? what about if the domestic and international terminals are in different layers in the same building?

What makes me think mapping the BRU connecter building as `aeroway=terminal` is reasonable, is that it is completely in the airside of the terminal, and took on partial functions to transfer passengers between airside (Pier A) and landside (main building).

I now understand that our core disagreement is whether the `aeroway=terminal` label is from the operational perspective or building property perspective. Your idea is that the tag describes the airport's operational organization, which could be so varied from airport to airport that it would be hard to establish a unified worldwide 
understanding in OSM. My opinion is that the tag describes the building function of transferring passengers between the airside and the landside, and it's the currently used one in practice. I don't deny the reasonability of your point of view, but it's better to keep following the mainstream practical usage.