Here is how I install Steam on Debian:

sudo dpkg --add-architecture i386  
sudo apt update  
sudo apt install curl  
curl -s http://repo.steampowered.com/steam/archive/stable/steam.gpg | sudo tee /usr/share/keyrings/steam.gpg > /dev/null  
echo 'deb [arch=amd64,i386 signed-by=/usr/share/keyrings/steam.gpg] http://repo.steampowered.com/steam/ stable steam' | sudo tee /etc/apt/sources.list.d/steam.list  
sudo apt update  
sudo apt install steam -y  

Edit: Added a fancy block.