There are a number of pressure treating methods used today. The one I have read the most about is the typical "pressure" process by which lumber is placed in a cylinder and it is pressurized with the chemicals inside.
But that is a general description for what really has a number of different kinds of pressure treating processes. The ones I have heard about are the a full-cell process, a fluctuating pressure process and the Boucherie process.
That is the extent of my knowledge. You will have to research further at your end as technology continues to evolve.
I won an inflatable 6 person inter hot tub. I have a large deck. 12x 20 on both sides of the house. With a header to the house and the deck was built 6
I'm putting a swim spa into an inground concrete bunker and will build a deck around it. I have been advised to keep the deck joists a minimum of 6 inches