Usually it's loaded directly to /sdcard. It will delete it once it's used.
You can also just download the update directly (onto your desktop or wherever) if you like.
ONYX/Nova3/Nova3:10/2021-11-30_22-32_3.2_5bccd60/4167:user/dev-keys
http://firmware-us.boox.com/cbcb14e1...4d0/update.upx
ONYX/Note3/Note3:10/2021-11-30_21-13_3.2_5bccd60/4165:user/dev-keys
http://firmware-us.boox.com/ef1f3fc1...f1c/update.upx