Short answer
Non-EU citizens generally need a D2 entrepreneur visa or a D8 digital nomad visa before registering a business or freelancing legally. EU citizens can register more freely. In practice you register a company or open activity as a sole trader, get a NIF and register with Social Security and Tax Authority. Rules and requirements change, so confirm with AIMA and a local accountant before committing.
General information, not legal or tax advice. Rules and amounts change: confirm with official sources.
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