How To Flagstone

This is for a "highend" new backyard (no existing landscaping) in a "highend" neighborhood. I am told there are two ways to do the flagstone:
* the same way as pavers, with sand layer underneath, the pavers set, filled in and swept with sand, and watering it down when done? or,
* with a concrete layer first, then glue the flagstone with mortar and filled in with ???

**sealed or not sealed with something?

*** should it be attached or not attached to the slab foundation?

**** How far up the foundation (or down from stucco) should the level patio top be?

***** Can it be DIY? Or I need above info to find out what method the contractor should use (one told me uses sand method)

The yard is large, so there may be one or two patio areas, a back and a side. The back patio may be 20ft x 30ft or larger.

Also, the area is east bay california (some seismic activity). The soil is not soil, it's like clay when wet.
One contractor I am considering is an older guy "just starting out" on his second career. He said "flagstone patios are easy" and would use the set in sand method. The other far more expensive contractor works only with a team of two hires - he seems to use concrete mortar.