To (re)pair the Steam Controller (2015) you have to open Steam app, go to Settings>Controller>Add new controller, then to pair it with dongle, hold X then press Steam button. You might also need to switch to Dongle mode first by holding A then pressing Steam button.

To pair it via bluetooth hold B press the Steam button.

Pairing on Raspberry Pi (tried 4b+) didn't work even though steam-devices package was installed.

• rpi 4b+
• https://media.steampowered.com/steamlink/rpi/bookworm/arm64/steamlink-rpi-bookworm-arm64-1.3.32.316.tar.gz

• Steam official docs archived

• https://www.reddit.com/r/SteamController/comments/hf7o8e/raspi_retropie_steam_controller_fully_working/ web archive link