Thank you for the answer.
I found the files for the settings on Windows:
arduino-cli.yaml and settings.json
If I understand it, you are changing the main settings and start the IDE, right?
On the old IDE I was able to start it with custom settings, without changing all settings for the other projects.
To be more precise, I had some custom settings (mypreferences.txt) in my project folder and merged them with the current settings (preferences.txt) in a temporary folder (temppreferences.txt) and used that to launch the IDE.
In this way, I was able to set different preferences for each project but leave the default settings when I started the IDE directly (without opening a project).
I assume, this is not possible anymore without modify some files inside the Arduino folders, right?