No, not all start as light novels. In many cases the light novels come afterwards as a way to capitalise (eg Demon Slayer, the manga finished a while ago, so while the anime is still running light novels are coming out).
Apothecary Diaries started as a light novel, but JJK, AoT and many others start as Manga
Apothecary Diaries started as a light novel, but JJK, AoT and many others start as Manga