As long as you are not prohibited and over the age of 18....
Yes, you can order EVERYTHING you need and have it shipped to your house. This INCLUDES the lower IF you wish to build from an 80%. Depending how handy you are... But it is NOT hard to mill even with limited tool working knowledge.
Option A: Find a local FFL and buy a stripped lower. Wait 10 days. At this point it MUST be built Featureless OR Fixed Mag.
Option B: Buy an 80% lower, buy a jig, buy a router (or drill press) and mill your own lower. Again it MUST be configured featureless OR fixed mag AND you must apply to the CADOJ for a long gun serial number to be afixed to your lower (engraved).
Option C: Buy a Ghost Gunner personal CNC machine that will mill the lower for you. Still legal as it is YOUR equipment. However, this option is very pricey as the machine is around a grand or so USED. around 1600 with a wait time for new.
DO NOT CONSTRUCT AN AR OUTSIDE OF A FEATURELESS OR FIXED MAG CONFIGURATION. If you configure it other then Featureless or Fixed mag, you are in possession of a Assault Weapon that is a felony according to CA Law.
Everything else as far as parts, are just that, PARTS. Uppers, Lower Part Kits, etc etc etc. The lower is the controlled as it is the firearm. You CAN legally build your own from an 80%, however, the above applies; Featureless or Fixed Mag WITH a CADOJ issued serial number engraved to it.
Home built 80s can NOT be sold once milled! Once milled they become a FIREARM. You must use your OWN owned tools. Someone else cannot mill it for you.
There is MANY options out there of vendors who still will ship to Cali. Others will not. AND i cant blame them with attempting to figure out what the law is this week.
Watch for deals. You can build an AR for around 400. Is it top of the line, no. But do they function and are accurate, YES. Ive built a couple 'budget builds' that shoot JUST as well as my factory AR I purchased YEARS ago.
Watch the EE for people with used but not abused parts. PSA will suck your wallet dry, youll see. :)
You will need basic tools to put your rifle together. You can find a punch set at Harbor fright for cheap that makes life easy for installing pins. AR armor multi wrenches are cheap. Vice and vice block make it easier to put your AR together but is not necessary to finish it.
Ask questions, read post, watch youtube how to assembly videos. AR's are like the Honda Civics of the gun world. There are aftermarket parts EVERYWHERE. Pay attention to milspec vs commercial. You want milspec!
Once you build one, you will want to do another!
I think i covered the basics.....
ETA: I did forget to mention if you go 22LR, Rim fire is not considered an AW. Also a NON simi automatic configuration is NOT an AW.