u/Due-Bell-6116

What are these commonly used 3D connectors called, and how can I most easily model these in my FreeCad or elsewhere to incorporated them into my own models. Are they standardized in anyway? They look like a kid's wooden train tracks. I've downloaded a couple of models that use these, and I like the tight connection often produced. Google search really hasn't helped me on this. Thanks for looking.

https://preview.redd.it/j6564p1he0yg1.png?width=692&format=png&auto=webp&s=f1714add3771c9a3f18982ce89b643014f3c21f0