The previous suggestions about too many open tabs to keep track of is the first thing. Bookmarks in your browser are an option. Otherwise, a "watch later" playlist in YouTube is good if you might want to pick up where you left off on a different device.
As for the video randomly playing after the one you intended to watch, the screenshot below highlights the autoplay toggle. If you have it in the left (off) position, YouTube won't pick the next video for you and play it.
If you do decide to change this setting, keep in mind it may not take effect on any of your other currently open YouTube tabs.
As for the video randomly playing after the one you intended to watch, the screenshot below highlights the autoplay toggle. If you have it in the left (off) position, YouTube won't pick the next video for you and play it.
If you do decide to change this setting, keep in mind it may not take effect on any of your other currently open YouTube tabs.