No, they're not "sub versions", they are separate versions.
7.0.0 is one distinct version. 7.0.1 is the next version. 7.0.2 next, and so-on. These numbers are usually talked about in terms of "major", "minor" and "point" releases.
A major release usually represents a massive structural change (or a major rebranding exercise in some software). A minor release is usually some improvements, new functionality and often changes to existing functions. A point release is very rarely anything more than bug fixes or minor enhancements that don't change the shape of the database or functionality.
As a general rule, you should choose the latest point release for the major.minor version you choose.
In your case, you should upgrade to 7.0.latest, then, as that's out of support, make a choice to go to 7.4 or 7.5 (again, use the latest point release)
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