It’s quite a bit more complicated than that. Those joint ventures were for traditional cars, and even today Chinese automakers aren’t that great at making internal combustion engines. But in EVs, the Chinese didn’t get significant help from joint ventures; their EV industry predates foreign involvement (Tesla only set up there in 2018, a decade after BYD already started making their own EVs).
Sure, the Chinese probably picked up a lot of general industrial know-how from foreign investors, but a lot of their EV expertise is also based on figuring things out for themselves, and seizing an opportunity to exploit a new technology and disrupt slow moving incumbents.