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Definitely downloading and uploading company documents to people outside the company counts as stealing...

As a sibling comment mentions, it depends.

If you want to learn more, there is a thing called clean-room development which is a process used to reduce the legal risk of copyright and intellectual property violations.

No, that would be your government or employer preventing you from using your specialized skills to earn a living. In theory, a competitor can hire you for your skills - but your new employer will make it clear they are "only hiring you for your skills, not your proprietary knowledge". I've see that clause even as a junior employee.

There is not exact definition. Real world is messy.