When the EU [1] implemented these systems they announced it years in advance, delayed the deadline due to Covid disruption, and it's still optional — the alternative is for the receiving post carrier to charge the taxes and an administration fee.
But using this system, I can order something from Ali Express for €10 + €2.50 VAT, pay Ali Express €12.50, and they send the VAT to Denmark. The tracking number on the package proves the VAT was paid, and the package sails through customs.
(There's also a UK system, very similar, but I have forgotten the name of it.)