Migrate Firebase Dynamic Links to Redirection

You can use Redirection on any domain where it is running, including a subdomain, and it would function exactly the same. What problem are you expecting?

1. I have 30 important short URLs;
   would love to import the takout.csv from Firebase directly into Redirection.
2. How do I have to configure the DNS and Redirection to let
   subdomain.mysite.de/shortname
   work like before?
   (A record for subdomain “go” points to Firebase at “151.101.65.195” at the moment)

I can’t tell you how to configure the DNS – this is outside the scope of Redirection and specific to your setup. There are some details here: https://redirection.me/support/redirecting-another-site/

The supported CSV format is shown here: https://redirection.me/support/import-export-redirects/