Hi Ricarda!
Thanks for reaching out to us! Let me try to help you with that.
First, let me note that we are fairly limited in how much support we can give for custom development questions like this.
That said, we always like helping out and at least point users into the right direction as best possible.
We have a great article that will get you started on customizing our plugins > https://theeventscalendar.com/knowledgebase/themers-guide/
I’m not very familiar with the FacetWP plugin and have never seen it working with The Events Calendar but if you bump into any questions while building that customization, feel free to ask and we’ll try to help as much as we can.
Let me know how it goes and I’d be happy to help.
Best,
Victor