iPhone 7 Plus video is grainy and noisy

I have a brand new iPhone 7 Plus that has a fully updated iOS. I bought it for work explicitly because of the camera and how great it was deemed to be. However, any darker areas of the frame, or any clothes or materials that are a dark colour tend to fill with jittering grain and noise - even in brightly lit rooms. Sometimes it looks washed out as well. Focus and exposure were selected for the people in the shots. The low light capability has an unusable amount of grain. The dark areas of my videos do not have any of the fluid colours that I've seen examples of. To me, it seems like the ISO is too high, but there's no way to control it.

Test1: https://youtu.be/pm_-ozzquX4

- grain in the back of the seats, under the table, in the dark clothing

Test2: https://youtu.be/WnnkH8R_ToM

- grain in the hallways,e even where light hits

Test3: https://youtu.be/2q239v_8WKw

- brightly lit room, yet noticeable grain/noise in the dark clothing

Is this just how the camera is or is there a software or hardware problem?