This is one of those things where different countries take different approaches (or at least have different pointes of emphasis). But the classic example is a security camera you install on your own house (or these days, a doorbell camera). If it can only see your own property, that's a household activity and is not covered by GDPR. If it records video of people walking on the public sidewalk outside your house, then GDPR applies to that recording.

As noted elsewhere, most of the fines against private individuals have been issued by Spain. And my mediocre Spanish literacy leads me to believe several of those fines are for posting videos on social media.