Is there such thing as Multiple "versions" of a document in one?

No. Pages is a word processing application and cannot marsupialize multiple pages documents within one document. Not aware of any word processing application that has that capability. Pages does support multiple open Pages documents as tabs in the same window (see Pages Window > Merge All Windows).

This will come in handy: Pages User Guide for Mac - Apple Support

When one first saves a Pages document, that activates autosave and versioning. In the latter context, one can select File menu > Revert To > Browse All Versions… One is presented with a time line, not visually identifiable version numbers, that allows one to select older version content of the previously saved document.

See Restore an earlier document version in Pages on Mac - Apple Support

Pages lacks a proper outlining feature unlike Microsoft Word or the free LibreOffice Writer applications and has no means to differentiate draft from other documents. Pages does have a built-in equation editor that assumes that you know rudimentary LaTeX or MathML.

See About LaTeX and MathML support in Pages, Numbers, and Keynote - Apple Support You want the blahtex 2.22 PDF manual whose provided link shows what LaTeX restrictions exist in the equation editor.

And Add mathematical equations in Pages on Mac - Apple Support

You’re looking for something closer to a revision control system, particularly if you want the ability to revert to earlier versions. Pages can undo / revert to earlier versions too, but probably not the way you want or need.

Maybe an app such as Scrivener, or an alternative, or maybe a mind-mapping app will work here.

Closest Apple app is probably the Journal app included with iPhone, and that’s not a close match.

There may be (are?) educational apps that can provide something similar.

Pragmatically, a Python or zsh script, or Shortcuts, or a Swift app customized to create your worldview would itself not be difficult to create, and is probably the best approach. This also gets rid of all the complexity around creating tabs and such, where the user gets a visual guide to how this all fits together, and can tap on and launch each component app of your worldview. Underneath, it’s managing a folder with Pages files, and with some templates.

MaineApplePie wrote:

It is called a document merge for Google Docs.

I am not particularly knowledgeable in Google tooling, services or apps. That tooling just isn’t used around here.

Based on a cursory look… If you can’t easily unmerge them, and can’t continue to manage and update the documents separately, then what you are seeking is available in many tools.