Just agreeing to it isn't necessarily enough at least in some countries.
2) What laws and sections specifically makes that possible? Are there examples of that happening?
3) Where can I find that interpretation of article 50(?)? Some other article? (To the extent that things would need to be labelled/watermarked etc.)
No. Here is the list of organizations that have the power to make laws in the EU (and JUST the across-the-EU part of that list, within countries, within states, within provinces, within towns there's another list). This is referred to in legal tradition as the "Hierarchy of norms", because there is also a clear order defined.
https://eur-lex.europa.eu/EN/legal-content/glossary/eu-hiera...