It uses the entire sensor and interpolates, unless you tell it otherwise (I think it's labeled "smart teleconverter" or such). Whether that improves high ISO performance or not is something *hotly* debated, but general consensus seems to be it doesn't do jack or, if it does help, it's less than what a half-decent NR program would do with a full-resolution image.
|.....the ROYAL LOTUS 2017/08/25-NEW YORK..... by Chiwat|
from Wild flowers
|Coffee and Mango cake by clicker88|
from Another cup of coffee