Copying these over from other posts so hopefully they’re easier to find in the future
Jump+ (app, Shonen Jump manga mostly)
LINE Manga (a variety)
YanJan+ (Young Jump manga)
Comic Days (Afternoon/Morning/etc brand manga)
Bookwalker (often has free to read)
This site specifically shows where to buy digital copies of JoJo, but some of the 45+ online retailers listed when you scroll down probably have free chapters or volumes of Naruto