[MinimalAction]

That blackbox being their entire moat, I would assume they'd never want to open-source any function. Mathematica as a front-end has innumerable frustrating bugs, but its CAS is top-notch. Especially combined with something like Rubi for integration, for me nothing comes close to Mathematica for algebraic computations.

[eulerboiler]

Many functions source ate viewable. Use https://resources.wolframcloud.com/FunctionRepository/resour...

[gdelfino01]

Many built-in functions are open source too. Use the "PrintDefinitions" ResourceFunction to see the code of functions that are implemented in Wolfram Language itself.

[mkl]

Source available? The license is still proprietary, right?

[LegionMammal978]

Yes, it is all proprietary, but there are still ways to inspect most of the WL-implemented functions since the system does not go to extreme pains to keep them hidden from introspection. It is not unlike Maple in that sense.

[oh_my_goodness]

Yeah, hiding the recipes for how the math is really done would make the whole system kinda guess-and-hope for serious users.