If I recall from digging through the code before, it mentions using a modified kernel to simultaneously interpolate and antialias. I don't know which methods are relevant to that chunk of code, but you're free to look at the code yourself.
I can't really point out line numbers since it changes between versions. Circa R2015b, most everything is still in imresize.m, which is in the IPT directory. Look for a function called contributions().
By R2019b, the parts of imresize() are spread across several files in $MLROOT/toolbox/matlab/images/+matlab/+images/+internal/+resize/
Look in that directory for contributions.m and see if you can make sense of it.