For example what is correct "My car is getting repaired" or "My car is being repaired" , can't understand the difference , the first and the second sentences seem to be narrating about a process rather than state
Both sentences are correct and convey almost exactly the same information. I can't think of a situation where you could use one but not the other.
The only difference I might perceive is that "getting" feels slightly more "active" while "being" feels slightly more "passive," but this difference is very small.
Both are correct, and both are passive. The difference between active and passive is that "the car" doesn't actually do anything so it is the object of the sentence and not the subject. Passive is created with the verb "be" (so "IS getting"). The sentence becomes inverted, and instead of the subject going first (like " The repairman repaired the car" where "the repairman" is the subject and the car is "the object") the object goes first (so the sentence ends up like "The car gets repaired by the repairman").
The get passive can't be used with stative verbs. For example "Believe" is a stative verb. In the active voice we say
I believe you.
Or in passive:
You are believed by me.
But we can't say
You got believed by me*.
Some particples indicate a state:
John is married (Married is very much like an adjective describing John's state)
John was married last year (Unclear, is this John's state last year, perhaps before a divorce? or was the wedding last year)
John got married last year (Clear. This must be an action, and so it means the wedding was last year.
The fact that the "get" passive forces an active and specfic meaning to the participle means that it tends to be used for particular actions with surprisingly positive or negative outcomes:
She got promoted!
I got fired :(
I got told off.
You could use "was" in all three examples, but it would lose something. Using "I was promoted" is a boring telling of a fact, rather that Exciting News
In some cases the "get" passive becomes so common that it can be called a collation or even an idiom.
I'm getting hungry! (You would hardly ever say "I'm becoming hungry")