Yes, Nokia 1 does have the Google Assistant in-built.
Nokia 1 is not completely metal body. It has metal chassis frame and the housing is made of polycarbonate material.
No. Nokia 1 does not have the front camera flash.
Nokia 1 has single core ARM Mali-T720 MP1 GPU, which has 550 MHz frequency.
The Nokia 1 is an inexpensive Android smartphone, and as you can tell from the Nokia 1 review, you very much get what you pay for.