SharePoint allows you to create hub sites in your environment to help with searching and content roll-up. You can then attach existing sites to that particular hub site. However, you haven’t been able to associate a hub site with another hub site. Microsoft has addressed that issue, and now hub sites can be associated with another hub site. Here’s how that works…
In the Hub site settings of a SharePoint site that’s been designated as a hub site, you now have the option to associate that hub site with a parent hub site:
It’s also easy to add links to the associated hubs as well as any hubs that are associated with this hub site:
When you search, the search breadcrumb path at the top of the page displays the associated hub where the content is:
There’s a lot that goes into determining the hub site structure, as well as how you might want to associate hub sites to other hub sites. I suggest taking a look at this: Associate a SharePoint hub site to another sub to connect search experiences.