For products covered by the legislation listed in Article 4(5), a product may be placed on the market only if there is an EU-established economic operator responsible for the Article 4(3) tasks. That role can sit with the EU manufacturer, importer, authorised representative with written mandate, or in some cases a fulfilment-service provider.
The tasks are concrete: keep the declaration available, ensure technical documentation can be supplied, cooperate with authorities, notify risk when you have reason to believe one exists, and ensure corrective action is taken.