How do you lot set your default search engine google chrome?

On a phone it's inside Chrome itself - open Chrome, three dots, Settings, Search engine. Not in the phone's main settings, which is where people usually go looking.

Three dots top right > Settings > Search engine, then pick Google in the dropdown. If it keeps switching back on its own, that's nearly always an extension doing it, go through chrome://extensions and remove anything you don't remember installing.