The EU Commission has published fact sheets in relation to the new rules, please find the links below, they are very useful.
Import OSS :
https://op.europa.eu/en/publication-detail/-/publication/77255bf8-65cd-11eb-aeb5-01aa75ed71a1/language-en
OSS for sellers :
https://op.europa.eu/en/publication-detail/-/publication/a8580815-6507-11eb-aeb5-01aa75ed71a1/language-en
OSS for interfaces :
https://op.europa.eu/en/publication-detail/-/publication/5a174f57-65cd-11eb-aeb5-01aa75ed71a1/language-en
Image source: http://unsplash.com
Michaela Merz 