Firstly it depends on the budget, then the process is decided according to it.
As in developed countries like US getting junk cars from junkyard and blast then doesn't cost much but in small budget movies they replace Costly car with a cheap car and then they blast it. Like in the following example of Race 2
Where you can easily spot that two different cars are used, one for show-off and other for the Blast purpose.
Similarly toy cars are also used in small budget movies which looks like real car but they are not. Similarly toy cars are mixing with the real car with CGI and becomes good option.
One more example from cracked.com is here-
In The Dark Knight -- The Big Chase Scene
Two specific points during the insane car chase at the halfway point
of The Dark Knight are so over-the-top they seem like they would have
had to be computer-generated, if for no other reason than they would
have killed the stunt drivers.
The first is when the Batmobile first shows up to take out the Joker's
convoy. It speeds in ...
... and offers what equates to a vehicular uppercut to a garbage
The scene ended up in the trailer and inadvertently encouraged a bloat
of fanboys to flock to their keyboards and pound out protests against
the fake-looking CGI in the movie. But as Christopher Nolan has proved
time and again, he doesn't mess with that shit if he doesn't have to.
No, what you are actually seeing there is a complete one-third-scale
model of the Batmobile, the garbage truck and a large section of lower
Wacker Drive in Chicago.
They stuck the two vehicles on a guide and smashed those mothers
together. What you see in the film is the result.
Even the damn 180-degree move that the Batmobile pulls off at the end
was done by a radio-controlled model.
So what about the climactic moment in that scene when they flip the
Joker's 18-wheeler after Batman clotheslines it with a grappling hook?
If that was a model, it was pretty goddamned convincing.
The flipping of the semi was accomplished with a technique known in
Hollywood as flipping a real goddamned semi. To get the mind-boggling
amount of upward force needed to lift the big bastard head over heels,
the FX crew built a huge steam-piston mechanism in the trailer.
For more details click here