Both my Pentaxs (K-3ii and K-1ii), and I think both my Olympus (M5 and M1). I'm not sure on my ancient Sony (NEX6) or my even more ancient Phase One. Its called something like Long Exposure Noise Reduction (LENR) or Dark Frame Subtraction.
Sony a7 is a recent camera and has Long Exposure Noise Reduction activated by default for shutter speeds of 1 s and longer. You can disable this feature.
@Les Sarile might be right about choosing film over digital for night photography given a lot of non-trivial manipulations involved in boosting the sensitivity of the digital sensor.
I recently started doing street photography in darker settings.
I am using a Zenit-E with a helios 58mm f2 lense. I believe i used 1/30 for exposure with open diaphragm. For film i used Portra 400. My question is, why i get so much general background noise and how do i choose better camera settings or film for this kind of photography? and how to reduce the noise since generally 1/30 seems to work otherwise (picture 3)
As the saying goes, expose for the shadows when using most color negatives and b&w film. This means to make sure that you expose for the detail in darker areas that are important and the film will take care of the highlights. Kodak Portra 400 can handle a lot of overexposure. As a reference below, I overexposed Portra 400 +10 stops and you can see that with the mildest of post work what can be recovered.
In practical application, a frame of Portra 400 can usually recover 3 stops of underexposure too. In this example, I can recover both shadows and what would normally be blown out highlights.
Picture 3 has more and larger highlights than the others, so the automatic printing didn't bring everything up to "average" like the others. If you want good results either do it yourself or have a custom lab do it. I assume you still have the negatives - see if they'll print the rest using the same settings as picture 3.