Run Steam in gamepadui/big-picture mode at boot

create sytemd service:

/etc/systemd/system/steam-bigpicture.service

 

[Unit]
Description=Steam Big Picture Mode
After=network.target

[Service]
User=YOUR_USER
ExecStart=/usr/bin/steam -gamepadui
Restart=on-failure
Environment=DISPLAY=:0
WorkingDirectory=/home/YOUR_USER

[Install]
WantedBy=graphical.target