Run Steam in gamepadui/big-picture mode at boot

Pre config

You just need auto login an steam

Systemd-service

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

Or as auto-run script

Create /etc/xdg/autostart/steam-big-picture.desktop

Or: ~/.config/autostart/steam-big-picture.desktop

[Desktop Entry]
Type=Application
Exec=steam -gamepadui
Hidden=false
NoDisplay=false
X-GNOME-Autostart-enabled=true
Name=Steam Big Picture