Iphone 6S problem?

[RlCK]

So its normal? cause I got worried i was like HOLY SHIT i already used 8 GBs
I also remember my old iPhone 5s lasted for like 2 years and it was 16 GBs

Yeah completely normal. They advertise and sell the iPhone as 16GB but that does include iOS which already makes up for 2 to 3 GB.
The fact that the advertised '16GB' is actually ~14,9 GB - pre-installed iOS = ~12 GB available for the customer to fill in themselves.

If I need space I usually run a cache cleaner, cleans up about 200-500MB after a day of using my phone... can be quite a lot for 8-16GB devices.
How to do, for non-jailbroken devices: https://youtu.be/R9luXZAYEQ0?t=48s