Someone had a similar question the other day with regard to 力…You could also check out that thread: Newbie question about "power"
The way wanikani uses the terms:
Radical = part of a kanji. A lot of the names for these are made up by wanikani to make funny, memorable mnemonics.
i.e. 力 as a radical for power, can be a part of other kanji like 効, which is made up of 力 + 交
Kanji = character that represents an idea. 力 represents “power” and wanikani first teaches it to be read as りょく
A lot of the kanji in the beginning double as radicals since the characters are so simple and you find them as parts of other kanji.
Hiragana and Katakana characters are simplified parts of kanji, that’s how they were formed.
Edit: This is not really important to know, but this chart illustrates how hiragana and katakana were simplified from the chinese kanji: