I am just learning Unity and what I have learned thus far is that there is at least some level of modeling is possible using Unity instead of importing assets from something like Blender. My question is from an 'Architecture' standpoint. What should I build in Unity and what should I consider building in a 3d modeling program? Is the answer just based on 'what is possible in said engine' or is it better in some cases to go ahead and make that shack in Blender instead of Unity. question make sense?
You shouldn't really use Unity as a modelling tool at all. At most, you should use those internal capabilities to prototype things.
I want to follow up with another question Which graphical items in a game should be done in one vs the other (game engine)vs 3d model) Example: Characters and 'monsters' = 3d Moddeler Houses and buildings = ? weather = ?
Unity shouldn't really be considered a architecture program. If you want a architecture program here's a list of free and paid ones. Free: Blender Paid: 3DS Max Maya Maya LT Cinema 4D I recommend Cinema 4D because it is the easiest (in my opinion) to control and use.
Note that Blender/Sketchup import directly into Unity. There's no need to export. Blender is free for commercial use. Sketchup 2015 isn't, but you can maybe find an older "Google Sketchup" version which might be. Sketchup is easiest for architecture and anything "box shaped".
There's ProBuilder system/plugin/whatever that allows you to prototype levels by modeling from within the editor. Not sure what the price was ... $50 or more, IIRC. However, unity isn't made to be modeling application, you can either model entire level in external editor and export it OR you can make pieces of environment and position then within unity.
There's the free ProBuilder Basic, the full ProBuilder Advanced for $95, and ProCore Complete for $150. If you're heading down this route you likely want the full package as it's a very nice collection of tools.
this is all interesting but it doesnt really answer my second questions but that is ok I figure its a bit of an obvious answer
Honestly I feel like the answer to your second question is simply whatever gets the job done for you.