How to add Subdomains and Users custom domains to my vercel project?

Question

Im currently building an application using vercel and next js.

I want to allow my users to have an subdomain so user1.myapp.com and also allow them to have add their domain to my project so user1.com

I also have the problem that my domain has the TLD .de and vercel doesnt support them so i cannot transfer it to vercel. (Im also not sure if i have to do that so i realise my idea)

Does anyone already had this problem and knows how to fix it?

Best regards and thanks

I already read through Vercel Platform Startet Kit but im not sure if its what i need.