I just installed Radicale and could not at first figure out how to add a calendar that I could edit in the app. And since I could not find any decent post on the topic, here is how to do it (as of iOS 18.7.3 on an iPhone 13 pro, but I imagine the process being very similar on other devices):

1. Open the Settings application.
2. Scroll to the bottom and open the Apps options.
3. Open settings for the Calendar application.
4. Open Calendar accounts.
5. Add a new account.
6. Chose "other".
7. Chose "CalDav-account".
8. Enter URL and credentials for your Radicale server.

I have SSL set up with Let's Encrypt managed with Caddy so that was all I had to do. I say this because I read one discussion about SSL being needed.