is there a feature to auto create sub regions by just excluding air for a build like this?

[MrJoneshere]

[maruohon]

There isn't any feature to auto-create regions. The only ways to create regions are via the Area Editor menu, or using the addSelectionBox hotkey. I'm not sure if you specifically want to create regions, or if you want to know how to quickly/efficiently select a build like this into a region?

For the latter, as long as the build wouldn't be touching the ground or other nearby stuff (at least one full block gap from the lowest/furthest block of the build), you could use the selectionGrow hotkey to auto-grow the selected sub-region box around any connected blocks.

And also in general unless you are already using the Expand selection control mode, I'd highly recommend giving it a try instead of the default and kinda useless Corners mode. In the Expand mode you right click to reset/collapse the selection to the pointed block, and you repeatedly left click to expand it around all the clicked points. This allow you to always select a build without any temporary helper blocks, as you just click on the furthest block on any side (or corner, if there is one).

Since this is a fairly small build, how would you use regions here, assuming you mean you would want to use more than one region for this?
Unless it's beneficial for the build process, or some parts of this build are optional or alternatives for each other, I don't think it makes sense to divide something this small into several regions in a general sense. Sub-regions become beneficial in builds that would be like considerably over 50% air and somewhere around like tens or hundreds of thousands of blocks of air without using sub-regions. So for example big castles and especially world eaters and similar things.