Thanks to @RyanNHG and @DullBananas for such a swift answer!
Being a lazy programmer, I went with the shorter solution.
I tried to go to ‘innerspa.html’ using a Browser.Navigation.load "innerspa.html" in the update section and it worked like a charm.
Not too bad for a bunch of blunt fruits 
And I guess I learned a bit about http routing on Netlify, if I ever wanted to access the innerspa.html directly from a URL and not coming from my “outerspa”.