I've wondered if the phone uploads each word entered to a server, to get suggestions for the next word. That would be awful for data privacy. So I hope it's using a program internal to the phone.

Now I've checked the Keyboard settings perhaps for the first time, and found these options under "Settings - Language & input - Gboard", which can be turned on or off:

Text Correction - Next-word suggestions: Use the previous word in making suggestions

Text Correction - Personalized suggestions: Learn from Google apps and services and your typed data to improve suggestions

Advanced - Share snippets: Automatically share snippets of what and how you type in Google apps to improve Gboard