Hi Tommy -
I have some modest experience with landscape design, irrigation design and plant selection... Some of the things being asked here would require one or more of the following - hours of work, testing and or a site visit. Many of the questions you ask are large topics unto themselves. Ths is not laying out a raised bed or revamping a planter, this is a large scale design and build project you asking about.
With the irrigation requests... Are there enough? Lots of diffent kinds of sprinklers and they can require different pressure at head and have different throw distance. How many valves, where are the valves, what is the pressure at the valves. All this would have a bearing on the number and selection of the sprinklers and emmiters. California has drought issues, can you afford all the turf, do you have a water budget, in terms of both water and money? Some commuities HOAs limit the can limit the amount of water used.
While less knowledgable about soils, with the drainange questions, understood it clay, how well does it drain? How long for a 1'x1' water filled hole to drain? Are the drainage questions related to irrigation water or rain, or both? Say turf requires X" of water a day in that climate, how well does a inch of water drain... What is annual rainfall and roughly when it rains a daily average.
What kind of plants? What type of enviorment do you enjoy? Do you or do you not want to attract wildlife? Are you going to spend time in that area sitting - relaxing or is it more just a visual point of interest. Xeriscape - drought tolerent plants, a water trade off for the turf? You will need to consider hydro zones, i.e plants with like water requirements per valve, you don't want thirsty plants and drier plant on the same valve. Close neighbors, does some plants need to function as screening?
What I would suggest is identify a (build) prioritized sequence of steps. (This would be a viable thread in itself)
Water and monitary budget
Impact of drainage and sloping
What type of enviorment would you like... including budget considerations
Irriagtion...
Plants
Then identify what should be done by a professional
What you can do if you want to educate yourself by reading and learning (not just question on the internet)
What you can doing knowing what you know now, make up as you go, asking others.
It seems you have a nice fairly new house. I'm sure your glad your contractor did not rely on asking questions on the internet to build it. While a yard is a little simpler, it not that much different. Decide what you can - need to hire out. Decide what you can commit to really learning about and understanding. Decide what your comfortable winging and learning as you go.
While I did not answer your questions, I hope above helps you in moving forward and having a yard that meets the needs and you enjoy. Happy to answer any "in context and scope" questions you might have.
PS - Many books Sunset for example present a good primer of many above topics in addition to the general plant information.
PSS - Sorry for poor spelling
