How blockbusters movie producers deal with 8K videos editing
The answers are easily discovered by reading trade publications and reviewing videos posted by industry insiders.  Industry professionals don't keep secret things as simple as how they deal with 8K workflows. 
 Indeed, they talk about it so as to signal to prospective employees and customers that they are 8K-ready.

That said, your question is impossible to answer because it relies on false assumptions: I have an iMac Pro, and I can render 8K video in realtime or near-realtime (I don't have the most powerful model).

Your question is also vague, because 8K video can mean many things: it can describe the acquisition format, the exhibition format, or both.  Again, it is straightforward (with the right CPU/GPU/IO subsystems, costing less than $10,000) to render 8K source material to a variety of exhibition formats.  With the recently released "consumer" 8K displays (costing ~ $4000), Linus Tech Tips [showed how to hook up not one, but two 8K displays to a single GPU],1 further demonstrating that while it's not cheap, it's neither esoteric nor prohibitively expensive for anybody who can afford to spend sports car money on their computers.

Finally, and this may be the most helpful aspect of the answer to your question: proxy files remain the preferred way to scale video assets to gigabit network bandwidths and laptop levels of performance.  It is also preferred by VFX houses which may have hundreds or thousands of layers of textures, plates, effects, etc.  Compositing thousands of layers is not a real-time process, and there an 8K rendering pipeline can really gum up the works.  Again, the trade publications have many articles explaining the process for creating the proxies, applying the effects, and then rescaling to exhibition format.

As tools, techniques, and technologies are rapidly advancing, the best way to get the latest answer to such a question would be to search for "8k workflow".  Add "youtube" if you want to see a video that answers your question.  Add "vfx" if you care about that.  Add your favorite movie title and maybe that will hit as well.  For example: searching for "8k workflow vfx guardians of the galaxy" led to this article about Guardians of the Galaxy2, which covers in detail your question.