1) >How can this phone act as a "FULL NODE" of BTC? Is this really true? You could throw away your full node on linux computer, and instead use the HTC phone?
A smartphone is just a lower power computer that fits in your pocket. For a computer to be a full node, it just needs to run full node software. Phones can already do this, and it seems like HTC is just pre-installing an app to run a full node on this phone.
In fact, right now you can run a full node on a phone yourself using the ABCore android app.
-First, I have difficulty to understand how its going to store the hundreds of GB of data...
By using a high capacity MicroSD card. The phone will have a MicroSD card slot, and MicroSD cards can already have capacities as high as 512 GB. Alternatively, the node could be a pruning node so it doesn't actually store the entire blockchain history. Pruned nodes are still full nodes, they just don't serve historical blocks to other nodes.
-I am thinking about the mining activity. how can this small piece of phone chip do mining if it cannot achieve high hash rates?
Full nodes are not necessarily miners. You can have a full node without mining or producing any blocks at all whatsoever.
2) Is such a fullnode phone helpful and positive impact on the overall btc network (what are the positive impacts?)?
Yes. Any full node is helpful to the network. Full nodes validate and relay all blocks and transactions they receive. More full nodes means more people are verifying that blocks and transactions are correct. More full nodes in general help make Bitcoin more robust and decentralized.