AI Setup → Generate. Three options: Gemma 3 1B (≈500 MB, the default — works on every supported device), Gemma 4 E2B (≈2.5 GB, faster on capable phones), and Gemma 4 E4B (≈3.5 GB, the largest).
Installing a generate model auto-deletes the previously installed one so you don't end up holding two multi-gigabyte files.