Where are they assuming you know it? It’s taught right there in that explanation.
Besides, that part is in hiragana, so you don’t need to have it memorized. They’re just enplaining the parts of the word that come together to make this vocab.
Well I’ve learned the word for ball, 玉, no problems there.
I was just wondering about the ~ねぎ part which turns ball into onion.
Thinking about it it’s not that uncommon that WaniKani is teaching me kanji with attached hiragana which obviously changes the meaning of having the kanji alone.
God help you when you realise that WaniKani doesn’t teach grammar or Kana only vocabulary.
The vocabulary means “Round Onion”
玉ねぎ
玉 = Round
ねぎ = Onion
Doesn’t take much of a mental gymnast to work it out; and hey… you’d have to learn ねぎ means onion at some point, it’s not exactly a rare word and is pretty much never in Kanji.
TL;DR You need to be studying kana only vocabulary and building vocabulary.