I want to clean up and add details to this building. It’s currently mapped as two separate building areas (linked below).
1 Way: Eitel Building City Apartments (92810541) | OpenStreetMap
2 Way: 92810561 | OpenStreetMap
The two separate parts of the building are connected by an indoor walkway that extends from floors 2-5 (5 being the top floor). There’s an outdoor footpath at ground level that runs perpendicular under the indoor walkway. How should this be mapped to not confuse any routing and make the most sense?
Given that these are both part of the same building (they share an address, the whole building can be navigated without going outdoors, the whole building is just apartments, etc.) it makes sense for it all to be mapped as a single building. Not sure how to represent though that the outdoor path does not phase through the building.
The other option is as it is now, to have it as two separate building areas. However, then we run into cases like how it is currently, where the two separate building areas have different tags because someone updated one and not the other. It also makes it look like they’re two separate buildings, which, as mentioned, is not the case.
I also might be overthinking this? Should it just be mapped as one building area and just let it be assumed that the footpath doesn’t phase through solid matter because that should be obvious and trying to worry about that is more detailed that is necessary?
The usual way to map this is using building parts (Key:building:part - OpenStreetMap Wiki). You would then have 3 parts that you can tag, and a main building area encompassing them all with their shared attributes (like postal address and name).
On how to map the connector, you can use building:min_level. See also Key:building:levels - OpenStreetMap Wiki for an example.
Thank you, additionally, do you have any indication as to how much it would break other apps if it was mapped as a building area without the building parts?
I often tend to map incrementally, so in this case I might combine the two building areas, then come back and add the building parts later.
Also what might happen or break if only one building part was added for the connector bit?
No problem, always eager to help 
You mean because of the pathway? You might get a warning that the way intersects the building. You can tag the way as a Tag:tunnel=building_passage - OpenStreetMap Wiki.
This mostly concerns 3D visualisations. If it isn’t ignored (which might happen), then it’s probably going to be some visual glitch. If it gets really bad, someone else might fix it for you.
To check how your work looks, you can check out OSM2World - OpenStreetMap Wiki.
If you are more broadly interested in 3D mapping, Simple 3D Buildings - OpenStreetMap Wiki is a valuable resource.
The usual way to map this is using building parts (Key:building:part - OpenStreetMap Wiki).
from looking at the map and the description (“separate parts”) these would also often be represented as 2 buildings, (a picture would maybe help)