Quick-fire questions
Is High Sierra’s APFS compatible with Ventura?
> Only backwards, so that Ventura understands High Sierra’s older version of APFS. There are many features in Ventura’s APFS that High Sierra doesn’t know about at all, so be careful when working with them on the same Mac.
How to create folders at the top of Macintosh HD?
> You can’t do this from Catalina onwards as Macintosh HD has become two separate volumes. The root level is on the System volume, which can’t be modified except by Apple, to ensure that it remains safe from malware. You can only add folders to the Data volume, which in practice means inside your Home folder as much as possible.