I’ve been noticing that every verb which consists of a kanji followed by じる such as 感じる, 存じる, and 禁じる all use the on’yomi reading of the kanji which leads me to believe that the じ in じる corresponds to another kanji but is always written in kana form. Is this the case and if so, which kanji is it?
じる comes from ずる (which you will also encounter) and that comes from する. Yes, that する, so, not an onyomi.
Interesting, so it’s an old form of する verbs using Chinese readings of the kanji.