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★ Best answer
Be honest and straightforward: say that you worked in the informal economy, that you learned a lot but you're looking for stability now, and focus on the skills you gained during that time instead of justifying yourself. Mention that you understand the importance of being above board and that you're happy this job offer is legal and formal.
The best approach is not to bring up the whole story all at once in the interview - the interviewer won't even ask about it unless they bring it up, so if the topic comes up you can say something like "I worked independently during that period, without formal structure, but it was useful for developing autonomy and responsibility." The point is to not sound defensive or like you're justifying something bad, because working without a contract isn't a crime. If they ask you directly what you did during those years, be honest but brief - you don't need to get into the details of why it happened, just emphasize what you gained technically and why you're now looking for a stable position where you can grow within a company.
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