The way the law usually protects is via trade secret law. So the deciding factor is if you are later using information that was a trade secret. Trade secrets have to have information that isn't known to the public.
For the bridge example, just because they taught you how to build a great bridge, doesn't mean it was a trade secret. But if they had some sort of special sauce on how to build a bridge, it could be.
It's more complicated than this. They actually have to keep the information secret, usually through NDAs, controlling code, not letting people show it off at conferences, etc.
This sort of "mind" trade secret is hard to deal with. There isn't a clear distinction between talent/skill and information owned by your former employer.
For the bridge example, just because they taught you how to build a great bridge, doesn't mean it was a trade secret. But if they had some sort of special sauce on how to build a bridge, it could be.
It's more complicated than this. They actually have to keep the information secret, usually through NDAs, controlling code, not letting people show it off at conferences, etc.
This sort of "mind" trade secret is hard to deal with. There isn't a clear distinction between talent/skill and information owned by your former employer.