In the Video, Alex is explaining how to use these two words.
In general, ‘already’ is ‘déjà’ in French, and ‘yet’ is either ‘déjà’ or ‘encore’ depending on the context.
‘Yet’ is most commonly used in negative sentences and questions, with the present perfect:
I haven’t finished yet: Je n’ai pas encore fini
Have you finished yet? As-tu déjà fini?
Have you already finished? as-tu déjà fini? (I am surprised that you have already finished)