
Hi @Johnny Lam ,
To answer your questions:
(1)Per my test, it takes a while to take effect, about 2 hours, not immediately and we can't control when it is displayed since this is a background process.
As a workaround, we could add featured links to the SharePoint start page to make users access the hub site without following it.
(2)Associated sites will automatically share the hub navigation bar after associating , so it is more easier to access the hub site from associated site.
However, for your requirements, it's necessary to Add links for associated sites manually at hub site to make it easier from hub site to associated site.
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