Not sure if a fighter jet is easy to make, even countries like South Korea have taken two decades to produce the 4.5 generation fighter jet and they still can't

Not sure if a fighter jet is easy to make, even countries like South Korea have taken two decades to produce the 4.5 generation fighter jet and they still can't build their own jet engines, and the KF-21 is still on block 1. Avionics and radar seems to be very expensive and where fighter jet manufacturers can make their own to capture the margins but I'm questioning whether a founder with no aerospace experience is able to produce military jets with even more stringent regulation than supersonic flights