Beware the uncontrolled out of date material in the public domain. There have been a number of significant design changes including different pier build sequences, car parks, and there will probably continue to be changes to interior fit out even during the build. The latest official MAG fly-through does show the inside of the refurbished existing T2 building (which retains its glass atrium). Once the extension opens in 2020 the old building starts internal reconfiguration to complement the new part, plus a start on Pier 3 which builds out from the front of the old building.
The extension was previously to be +140% of the existing footprint but has been scaled back to around +100%. It will still be a rather large terminal.