Some weekend speculation about what is happening with Austin construction. (Note: speculation can be wrong).
South side tanks - if these are for paint we should see equipment to remove "Volatile Organic Compounds" from paint fumes, similar to what was built on the north end. If this is happening there will be plenty of time to spot it, as it was a slow build process, and they only enclosed the roof in that area after it was built.
I also think they might build new plastic moulding facility in this area as that was located next to paint on the northside, the only real change is that paint and plastics would run east-west rather than north-south at the north end. The plastic could be for outside parts of the Gen3 car including bumpers and possibly some of the other finishing plastic trim.
It makes sense for the final steps of the unboxed process to be located downstairs in that area - attaching the painted panels (cladding), attaching the bumpers, etc. The completed cars then enter the tunnel for a trip to the west side end-of-line.
The prior "assembly" step may be located in that area or close by within the existing factory footprint.
IMO most of the rest of the south extension is probably offices and computer rooms. The elevated columns may simply be to allow a "floor void" to run cables and utilities ... (also possibly for storage and computer equipment?) I don't think Elon is joking about sleeping on the factory floor during the Gen3 ramp. but sleeping in the nearby offices will be more comfortable. Having the last critical steps of the Gen3 process located close to the offices at Austin makes a lot of sense.
On the west-side near end of line 64 superchargers are installed, Joe has hinted that he expects 96 and are are early indications they may be getting ready to add more. Joe also expects some Megachargers for electric semis over the west side. I am hoping Joe is right. 96 Supercharger represents an approximate 2X the east-side charger capacity and that is an important clue about the west-side logistics capacity.