There is no definitive answer as this is constantly evolving as new laws are made. That being said, as long as your lower receiver is not marked with “California compliant” or anything of the sort, it should be legal. Polymer lowers are becoming more and more popular so it’s important to stay up to date on the latest changes in the law.
There is no definitive answer to this question as polymer ar15 lowers are relatively new on the market and there is no case law specific to them as of yet. That being said, it is highly likely that they would be considered assault weapons under California law, which prohibits the sale or possession of assault weapons in the state.