oohh....

is it possible for human ear to detect 0dB??

is tinnitus normal phenomenon or disease??

I think TS meant the Sound Pressure. See here ->Examples of Sound Pressure Levels

dB always has a reference value, just often this reference level is implied and thus not stated.
In this case 0dB is the human auditory threshold in air.

Noise, it's in all systems. What you describe is when a measuring device (ears in your case) has reached it so-called "noise-floor", a minimal point where the internal noise of a system has overtaken the external sense/input/stimuli levels.

Inherently all systems generate noise. Try closing your eyes when you sleep, sometimes you can "see" little color streaks, or if you hear a surprising sound you realize your vision flashes white even when your eyes weren't open. The reason we dream is theorized as the brain's inability to interpret a lack of stimuli, so the brain starts making assumptions due to a lack of input, and that can be called noise in a way too.

But hell, i'm an electrical engineer, not an neurological theory.