someone talk me through changing default search engine in chrome to google?

if it's a work laptop and the option is greyed out, that's not an extension - your company has set it by policy. only IT can change that one.

On a computer: the three dots top right, Settings, then Search engine, and pick Google from the dropdown. On the phone app it is the same idea - Chrome's own settings, then Search engine. If you set it and it keeps flipping back to something else, that is almost always an extension doing it. Go through your extensions and remove anything you don't remember installing, then set it again. If the setting is greyed out with a note about it being managed by your organisation, it is a work or school policy (or something nasty that installed one) and the toggle won't stick until that is dealt with.