Q:

How did Amix Group get this helicarrier?

In Deadpool,  

A big fight scene takes place in a battered helicarrier at an Amix Group scrap yard.  

What is the history of this particular helicarrier? And why in particular was it sent to the Amix Group scrap yard?

A:

It was decommissioned and sold for scrap, it's not a helicarrier, and it never belonged to S.H.I.E.L.D.
Quoting from  ScreenRant's on set interviews with the Director and Writers:
It was decommissioned and sold for scrap 

What happens is in Act 3, Ajax, our villain, is trying to soften Deadpool up for the kill, so to speak. So he lures Deadpool out to this scrapyard, essentially. And among the things in the scrapyard is an old kind of beat up, decommissioned helicarrier. So it’s sitting there and it becomes a big set piece, part of the fight, this massive fight between Colossus, and Angel Dust, and Negasonic, and Ajax, and Deadpool, and a ton of thugs. So it’s just a great playground.
  But this is like the twisted dark side of the helicarrier we’re used to seeing. It’s not gleaming, and shiny, and cool, and flying through the air. It’s been sold for scrap, essentially.
(source - ScreenRant's on-set interview with the Writers)

It's not a helicarrier, and it never belonged to S.H.I.E.L.D.

It’s clearly not a helicarrier… [Laughs] … Because that would violate the Marvel S.H.I.E.L.D. universe sort of thing. [...] to be fair, there are Helicarrier-like vehicles in the X-Men universe. It’s not just S.H.I.E.L.D. that has them. So I think it’s fair game.
  So I think the more we can bring that world into his world the better. But it has to be done in such a way that it feels Deadpool. So, what better way to do it than a decrepit carrier that’s being stripped for scrap? And it’s dirty, and grungy, and nasty. When you see the shots that are up on the flight deck of that ship, it’s obviously not up to S.H.I.E.L.D. spec in its design. It looks more like a World War 2 sort of technology with… some… turbo fans.
(source - ScreenRant's on-set interview with the Director)

