While the idea of furigana is indeed to help younger audiences learn kanji, there is no real rule to its usage and as jda95 said, is more an arbitrary decision by the author/publisher. Oftentimes books have furigana only for “harder” words, but even this is really just up to the author’s discretion and has no rules. As an example of this, when I first read the Mahoyo VN, the vast majority had no furigana (other than for special readings, like magic spells being read in engrish but written in complex kanji).
BUT for some unexplainable reason, there were very rare instances of furigana appearing for REALLY SIMPLE words. I don’t recall the exact words but it was something as simple as like 見る with furigana of み like wait, this is a super basic kanji why…I don’t know. It’s weird but yeah there’s just no rules as to when to use furigana.