Here we go with another newbie-question 
As far as my understanding goes, there is the possibility in RM to send http-requests. GA-Routines are also triggered via http-requests. Right?
With that in mind, shouldn't it be possible to trigger routines via http-requests from the hubitat? And if not, why?
Short answer: yes, if the apps are designed to receive those requests.
Slightly longer answer: there is a lot more going on under the hood than just the final request (authentication, etc.)
So basically yes but no. 
Thank you for that. Then I will go on and hope, that Google finally get's it's stuff together so that routines can be triggered by switches / door sensors etc. 