Surviving Mars - Linux (Arch-based like SteamOS is, too)-Client has no Launcher for DLCs started

Karbuziran said: The Game launches directly into the Main-Game without any Launcher to allow de-/activating DLCs

That is the same on all OSs and platforms. This game was released before we had our launcher. You can disable them in your Steam Client though.

You don't need to do anything to activate them though.  

Karbuziran said: So, that means all the DLCs are de-/activated from within the Steam-Client and no Launcher is necessary anymore for "Surviving Mars" to regulate anything for the Game?
Because I remember in 2019 after I had purchased the Game (when I had tried it out @ PDXCON2019), there definitely was a Launcher for it - which was a little "wobbly" in Linux, but it was present and worked more or less - even in Manjaro Linux.

This game never had a launcher of any sort. It always went from Steam straight into the game. I frankly have no idea what you are thinking of or remembering there!