Spoiler free version: Part 7 onwards is set in a parallel universe with characters along with somw characters who have similar names such as Johnny Joestar whose name is similar to Jonathan Joestar or Gyro Zeppeli who is similar to the Zeppeli from the Part 1-6 world. So its not a continuation of Part 6 but separate

SPOILERS DOWN BELOW THIS POINT

I forgot how to mark spoiler so . . . .

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. . . At the end of part 6, the main antagonist Pucci resets the universe, essentially causing the entire world to become a parallel version of itself. So all the characters you see in part 7 onwards are actually 5hr characters from 1-6 but their parallel world selves. And part 7 literally replaces part 1 of the previous timeline, so Johnny Joestar from the Part 7 universe( called Steel Ball Run universe for convenience) is Jonathan Joestar from Part 1