How do you frame an exterior wall over 8 ft tall?
You buy longer lumber (2x4, 2x6).   I can go in big box and find 10' lengths, and sometimes 12'.   Any decent lumber yard I use sells these up to 12' in high quantity.   

This isn't part of your question but I have worked on new homes over the past 20 years.   We have gone from mostly 2x4 exterior framing to 2x6.   Honestly unless you are in hurricane land there isn't a huge difference save for the fact of insulation.   

Some people say you must do 2x6.  No you must not.  You can insulate a house better from the outside rather than inside the framing (I am not suggesting no insulation inside framing - I am suggesting insulation in framing and exterior sheething in extreme climates).   

But but... I have had engineers that proved break points in framing with 2x4s and it is around 9-10'.   This is where you start seeing a degradation in foundation rigidity.   Sorry I do not have research for this but I saw this on multiple reports where we had plans for 10-12' ceilings.   So basically we were made to move to 2x6s.   

I wanted to mention that because in fact I think for most moderate houses in moderate climates, 2x6 is overdoing it.  But when you start talking about higher ceilings there becomes engineering needs.