RH 2020/Adobe Technical Communication Suite 2020 - linking TOCs between projects

Thank you so much for your help Jeff!

If you go the FM to RH route, you create a RH project that acts as a container for the FM content that populates it - either by import (a one-time thing; if the FM source changes, you have to create a new RH project & import again) or by linking (a "perpetual" RH project that detects and "scoops up" any changes in the source FM content).

Depending on how your content and the FM author's content worked together, you could go the route of creating merged projects in RH  (one project from your RH content & one from their imported/linked FM content) OR you could both create HTML5 output and then tinker with it to link to each other afterwards outside of either product. YMMV 😁

So how does RH create HTML with content from a FrameMaker file? Don't you just have an RH project essentially? So you cant have content from FrameMaker files and content from html files created in RH 2020 in the same project? Is that what we are saying essentially?

No - Framemaker is a long-form document authoring tool. It came out of the book publishing industry & produces excellent PDFs (as a strength). One of the more recent outputs is HTML - it used to be only possible to import or link (using the TCS) FM content to a RH project and create HTML from RH. The newer versions of FM have now started to incorporate the "guts" of RH into FM "under the hood" to be able to produce HTML5 output by itself.
There's no RH to FM route - only by copy/paste or RH > Word > FM or RH > PDF > FM & fixing up the formating would that work. It all depends on where (& how) you want to author and produce output. For example, for one of my projects where good PDFs are still desired (for offline help), I author in FM and import FM content into RH and create HTML5 in there. In another project, I author solely in RH and create HTML5.