They got rich the same way rich people everywhere get rich - by exploiting impoverished and disempowered labor to the max. The only difference with the Apartheid-regime (to whom Elon owes his riches) was that they were pretty overt when it came to who it was that got to be the impoverished and disempowered labor.
To be clear… that hasn’t really changed all that much in South Africa. But, then again, it hasn’t really changed in the US, either.