Moto G LTE (1st) gen - where are system version updates downloaded?
Location is /cache usually, but for explorers even with root enabled cannot browse certain locations (not sure if you are using systemless root, in which case ES doesn't get root permissions )

Booting into recovery and using TWRP file manager (Advanced tab) allows you to browse and delete the update file as confirmed by OP

Regarding the other problem of update installation messages popping up, one way is to identify and "freeze* those /system components responsible for searching updates. You would need to search for indicative names. On my device (Moto X Play), freezin 
configupdater,SEpolicy update,Motorola update services did the trick. I use ROM Toolbox to freeze . Other alternatives like Titanium Backup should also work