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I worked at a deep foundations company for many years. We installed those screw piles, helical piers, push piers, whatever you want to call them.
The tip elevation, how deep the pile goes, will depend on the quality of the soil and the size of the bearing plates. The installer will keep driving them in until a minimum depth and resistance is reached. Extensions are connected to the lead pier to make sure the whole pile is long enough and you have something to connect to.
Once everything is installed, you can frame out your patio, figure out what elevation the cap for the pier needs to be, etc., then the last thing you do is cut the pier to the right elevation. The top of the pile gets cut to the right elevation rather than having it driven to the right elevation.
I suggest you get a quote from a contractor to put these things in. They'll have the right equipment to get everything to the right depth and in plumb. Most importantly, if any problems come up, which they always do, a qualified contractor will have the tools and know-how to deal with it.
If you have any questions, feel free to PM me.
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