I downloaded the Pi app on my Raspberry Pi400 to download the modded Minecraft version there! Now to my question: How can I now open a server to play with my friends?
You can create a server on the Raspberry, yes.
Only the Raspberry Minecraft + would not be able to withstand a Minecraft server.
Better to use a site like Aternos.org and set up a free server over it
Then I would open a Java server, but I want to create a server for the Pi Edition!
Then just use this tutorial:
https://tutorials-raspberrypi.de/raspberry-pi-minecraft-server-installieren/