For OnePlus 7, it can't be said if it is Dust-resistance or Water-resistance as the OnePlus doesn't have any IP rating.
It is quite strange that the company doesn't go for the IP certification for its flagship phones. It does, however, says that the phones are water-resistance up to some extent.
But there is no way to know how much resistant it is.
Check its full specifications OnePlus 7
OnePlus 7 Pro comes with 8GB of RAM and 256GB of internal storage. Out of 256 GB, user can easily use 236GB of memory.
For complete information on the product check: OnePlus 7