Do the DC series have a connection on Netflix?

I would like to watch DC series and wanted to know if I might have to change there so that I understand it better.

I only have Netflix…

Yes, some have

I know that Supergirl and Arrow play together and in the future also The Flash = Arrowverse

My recommendation: It doesn't matter which series you start with, everyone is cool. I started with Green Arrow and then Supergirl. In some episodes they even meet (usually one episode per season)

The Arrow-Verse series are all bad.

Look at Titans, the series is not part of it and is better than everyone.