I thought the plural of "un" and "une" was " des", but the video link example says it is "de". Is that correct?

Hi Friend, 

If you mean in 'de beaux arbres'  -

Take a look at the following Kwiziq lesson which will illustrate this little rule -

https://kwiziq.learnfrenchwithalexa.com/revision/grammar/use-de-d-instead-of-des-in-front-of-adjectives-preceding-nouns-partitive-article

Hope this helps!