YouTube has a limit of 5000 videos in a playlist. So, the default 'Watch Later' playlist was filled up quickly for me.
Then I created a 2nd playlist named 'Watch Later 2' where I'm 3600 videos in. But now I've ditched this playlist for something new.
Instead, now I simply open a video & close it, to add it to my History. 'History' also has a search button now, but it needs the exact video title I guess, no typos.
Also, I have a total of 84 different custom playlists that I've created for different topics & God knows the total number of videos they contain that need to be "watched later".
Bit random, but the history search function doesn't require an exact copy of the title. However it does feel arbitrary what words work for the search and what words don't. I was searching for mods for a game for instance and I used the search term "mod GAME NAME" And like 10 videos showed up from like 2 months ago. With the video I wanted (which was more recent) no where to be seen. Then I tried searching for "GAME NAME" and it was the first hit. So idk man. The history search just sucks