So  I saw new AR lowers available in a Turner’s ad , what exactly is  the deal?
Do they need to be built by a certain date?
Really confused as to what happening now with the New Year and Law / Restrictions.
Any knowledgeable California Folks out there that can chime in ?

Lowers that are not banned by name are still legal to purchase under the new laws. They can be built as rimfires, bolt actions, single shots, featureless, or with a new law compliant mag lock such as the Patriot Mag lock or Franklin Armory's magazine (I believe it was called the DFM).

The new laws really only stopped the Bullet Button, Radlock, etc type of mag locks and had no real effect on the guns or receivers themselves.

Pretty much this, go featureless and drop some mags

Rimfire conversion sounds good to me.  So does 458 SOCOM.

Does the 10 round magazine limit also apply to rimfire magazines?

If you go to a rimfire rifle, do you avoid all the assault weapon regulations on the rifle, itself?  If so, can you have a pistol grip, detachable magazine, etc?

As of right now - yes rimfires are legal and don't require a magazine lock. You still need to follow all other state and federal laws, such as no full auto, short barreled rifles, etc. This can be problematic as rimfires can more easily take folding stocks, and their barrels will appear shorter than their centerfire counter parts. Remember that California measures in shortest fireable configuration, so a folding stock on some rimfires is enough to qualify as an SBR.

The magazine laws make no exception for caliber so the 10 round mag law applies to rimfires as well.

I foresee both a rimfire and a SOCOM in my future.