Often I have to join date finding surveys at Doodle. Now, I want to merge my Doodle Calendar with my NC calendar in order to keep synchronized all the dates blocked with Doodle on all of my devices which are using my NC calendar.
Is there an app doing this job at servers side?
If you can publish your Doodle calendar as an ics link, you can subscribe to it using the Nextcloud calendar app. A sync function doesn’t exist, because Nextcloud should be the one and only “save harbor” of your data 
You should think about replacing Doodle by the Nextcloud Polls app to keep control over your data.
For my own polls I am using only Nextcloud Polls. But there are some people out there in the universe who are using Doodle. When I participate in such a Doodle poll I would like to see the respectively scheduled timeslots in my Nextcloud calendar.