I think what’s confusing is your use of multi-level post type directories. The second level _index.md might be better an index.md. But without a test repo I wouldn’t know. I have quite some paginator use cases, but none where it goes into multilevels with _index.md.
Have a look at this thread here, the docs they talk about might give some hints - I remember though reading it in the final docs already too:
The pages are built on the following form ( BLANK means no value):
[SECTION/TAXONOMY/BLANK]/page/1/index.html => redirect to [SECTION/TAXONOMY/BLANK]/index.html
Now let’s dissect it a bit:
Reference, but the referent is missing, not mentioned in this section. Which pages would these be? No idea.
Built by whom? How? Where are they now, these built pages?
on the following form
What follows appears to be a list, not a form, but maybe it’s a template followed by examples of usage? Can’t really tell from context. Perhaps the writer meant “following the template below”? Or perhaps not?
and then we are shown the so-called form:
> [SECTION/TAXONOMY/BLANK]/page/1/index.html => redirect to [SECTION/TAXONOMY/BLANK]/index.html
Are we supposed to read that first line as [SECTION/TAXONOMY/]/index.html since we know that BLANK means no value?
I think => redirect to [SECTION/TAXONOMY/BLANK] is actually a comment, because it doesn’t make sense as an arrow function, but does this comment mean that [SECTION/TAXONOMY/]/index.html redirects to [SECTION/TAXONOMY/BLANK]? Who knows?
In short: This section talks about the way the index pages are built. It lists how the pagination links are named and explain, that page/1/index.html is redirected to the index page above because their content is basically the same.
Hugo is the one building the pages.
Much of the documentation is based on discussions here on the forum. You could give back to the community by rewriting them so that people with no previous knowledge of Hugo can understand them.