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.

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mkl an hour ago | parent [-]

Source available? The license is still proprietary, right?

	▲	LegionMammal978 an hour ago \| parent [-]
		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.