On-device needs Chrome 139+ desktop, ≥22 GB free disk on the volume with your Chrome profile, and >4 GB GPU VRAM (or 16 GB RAM). If the download never finishes, free up disk space; check chrome://on-device-internals.
Auto routing: inputs under ~6k tokens run on-device if Gemini Nano is present (private, free, offline); larger inputs or browsers without the model go to Gemini in the cloud. The badge on every result shows which path answered.