Isn’t this illegal/unconstitutional?

Yes. But the method in which campaigns such as YesCalifornia have been planning it, there would be a long and arduous process involving approval from congress or the states. Although long, this process would be peaceful as well as perfectly legal and constitutional. First, there will be a referendum in 2019 to see if there is local support. From there (assuming California votes in favor), a supermajority in congress or a supermajority in a convention of states will grant a constitutional amendment that allows secession.

Will this method be easy? No.

But will it be legal? Yes.