How do I turn off autoplay on YouTube so it stops playing the next video automatically?

There's a dedicated toggle, that little play/pause slider on the player. On desktop it's in the bottom control bar next to the settings gear; on mobile, tap the screen and it's at the top of the player. Flip it off and YouTube stops queuing the next video. The catch: it's per-device and can reset, so check it on your phone, computer AND TV app separately. On a smart TV it's usually in the app's own settings. For kids, YouTube's 'Take a break' and bedtime reminders (in Settings) are a harder stop than autoplay alone.

The autoplay toggle is right on the player itself (that slider icon), not buried in settings. Tap the video on mobile to reveal it at the top.

It's per-device, so turning it off on your laptop doesn't touch your phone or TV. You flip it on each one, which is why it seems to 'come back'.