Are there pre-made components for VueJS to actually build an own app?
I cannot use anything from the VueJS demo app as the components there all have hardcoded demo data in them.
Is there something where I can download the components for theme elements such as cards, dropdowns, navbars, etc. or do I have to create my own components?
Yes, mostly components are hardcoded this is done for a demo purpose, in your app you can replace hardcoded parts with data from your server.
For example, you can take a Widget10.vue component from folder src/components/widgets/mixed and replace static data in apexchart configuration with data from your server.