What happens when I open a Set?
You see the whole list before you commit to anything, with translations, genders and context sentences. If it looks worth your time, Add to Words drops it into your collection.
Tap any Set to open it. You get the full word list before you commit to anything: every entry with its translation, its gender where the language has one, and a context sentence showing how it is actually used. Scroll through and decide whether this Set is worth your time.
Happy with what you see? Tap Add to Words and the whole Set lands in your collection, ready to study like anything else you have captured yourself.
Words you already have are skipped rather than duplicated, so re-opening a Set you have taken before simply tells you everything was already there.