Unresolved

Car design in new season

How is the design of the cars determined when a season ends and a new one starts? 
I know the design gets worse but how is the new design determined? Does it decrease in percentage compared to the previous season or do all cars go down to one design that is the same for all cars

I recommend following the link below and checking this guide out, it’s a bit outdated but the info is all still relevant. 

https://igpmanager.com/forum-thread/45546

Thank you. Was heavy reading and calculation but think I understand some part of it