Short answer, the PC.
It's coming from the Voice Commentary Add-On extension for the Google Chrome browser.
The Pi, while visible in the video, isn't connected to the board when playing online. It's just another computer.
In the video, it is playing through the speakers on the touchscreen. If I were using the Pi, it would be coming from the Pi through an external speaker and being produced by the PicoChess App which is a chess application written in Python running on a Raspberry Pi 3. Side note... the Pi has 60 engines on it, including Stockfish 14 (Version 3).
I also use the PC apps: Fritz 14/ Fat Fritz 2 to play bots on the PC.
Most of my other DGT posts are under 'Had2Ask' on YouTube. I think this one was under 'Marc Hamilton'
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Custom game [unrated] against an opponent rated 259 points higher than me. Sometimes, better to be lucky than good...