Is the lowest level FEM solver and the strain/stress (material constitutive law) update code open-source?

[NINGYURICHARD]

May I ask whether the lowest level FEM solver and the strain/stress (material constitutive law) update code open-source? It seems that all the available codes are related to the api. And I can not find detailed files about the FEM calculation, the definition of constitutive law, and the updated algorithms of stress/strain. Thank you!