YouTube developers have implemented a very useful feature of deferred viewing . This means that if you started watching the video and then closed the page, then at the next entry you will continue to view from the place where you stopped.
To avoid a lot of false positives (after all, very often people voluntarily close a video and do not want to watch it) the feature is activated only if several conditions are met: 1) the length of the video is more than 20 minutes; 2) you watched more than a minute; 3) there are more than three minutes left. And yet, for this feature to work, you must be logged in to the site.
Very handy thing. Not every desktop player allows you to bookmark, and even more so to do it automatically.