I have a web application with a root site collection:
I have further host named site collections like this:
Note: "bu" is just a host header managed path.
Then I have sites under these site collections like so:
I have tried to use
Set-SPSiteUrl to add a URL for a site collection but I get an error message saying this can only be applied to the root site collection at https://sp.site.com.
Any other methods within or outside of SharePoint?