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No, not at all. I know where I worked there was zero code reuse across teams and games.

Games of this generation had code that wasn't intended to be supported or managed after release (no patches for those consoles) and so the code tended to be fairly horrible.

Yes. Especially when there are common libraries and you can reuse their names, structures or even entire decompiled routines.

There's a fair amount of folklore going around, but less direct sharing than would be ideal.

It varies. Nintendo for example has plenty of different teams and internal frameworks, but not every game uses them.