"The simplest way to keep a Google Account active is to sign-in at least once every 2 years." That's all it takes to be active. Google also lists a few examples of "activity" if you're already signed in:

Activity might include these types of actions you take when you sign in or while you’re signed in to your Google Account:

• Reading or sending an email
• Using Google Drive
• Watching a YouTube video
• Downloading an app on the Google Play Store
• Using Google Search
• Using Sign in with Google to sign in to a third-party app or service

Other "inactive" documentation on Google's support system lists 'Android check-ins' as a viable form of activity. So theoretically plugging in an Android phone would be enough to keep your account active forever. It also says paying a Google One storage subscription counts as "activity."

Interestingly, Google Photos seems to have its own entirely separate "activity" metrics that could result in only your photos being deleted. Photos requires visiting the Google Photos app or website, or uploading a photo every two years.

People who are already over the "inactive for two years" mark won't have their accounts deleted immediately. The Great Google Account Purge is officially scheduled for December 2023. Having your account deleted shouldn't be a surprise to anyone, with Google promising to send "multiple notifications over the months leading up to deletion, to both the account email address and the recovery email (if one has been provided)."