One of my grandfathers was illiterate as well. Not a finance whiz, but not stupid either. He was a rural blacksmith and he was rather good with engines and small appliances, repairing them for paying customers.
This is also a good moment to meditate on "right of repair". As recently as approx. 1970, most of the machinery in daily use could be repaired by a skillful, even though illiterate, person. The advent of semiconductors did away with that and nowadays even highly qualified people struggle to update a firmware against the vendor's wishes.