I do not know if this applies to the β version as well, but the current stable release 11.0.1 is downloaded in /private/var/folders/zz. There, you will find a bunch of folders with weird names: in order to figure out which one it is, I would suggest to run sudo du -sh */, which will show the size of each folder: the largest one should be the OS update. As a side note for all out there, if, like me, your loading bar is stuck to 100%, but the activity manager definitely shows that it is downloading something, going directly to the folder in which it is cached and querying the size of it might be helpful.