Fireplace hearth installation? Before or after false chimney breast construction?

[Springingforward]

Hi all
I'm having a false chimney breast added to my lounge to contain an electric stove, oak beam and shelves either side. My question is, does the hearth get put down first and then the stud work gets built on top or is the stud work built and plastered first and then I insert the hearth in a t-shape?

I'm drowning in Google image searches 🤦🏻‍♀️

As it's not a real hearth I don't suppose it matters

But the people building the dummy breast need to make it to fit the hearthstone perfectly, which will be easier if you put it down first. Protected against drips and splashes of paint and plaster.

Thank you everyone for your help. After all that I decided to get the chimney breast done first as it ended up being a different dimension to what I thought. So the hearth is now going to be cut to a t shape.