Let me see if I can add a bit of information. I have rarely used bookmarks here on the forum, but I did have a few, which are now gone. I have limited expertise in computers, but I'm not bad at knowing what I do/don't know! Here are my "findings", but anyone who sees that I'm wrong should speak up. We'll see if we can get some better guidance.
For starters, we seem to be mixing up forum bookmarks with browser (e.g. Chrome) bookmarks, also known as "Favorites" in Internet Explorer:
Two lines above that there is a blue star icon. If I click on it, I get the message: "To get your bookmarks on all your devices, turn on sync."
This is the bookmarking of Chrome - your internet browser - not the forum (which is why Google wants to sync them on your devices).
Everything
above the blue band of the forum that starts with "2021 Camino Guides" on the left and goes to "Search" on the right, is a function of Chrome (not the forum). You should still have any of those bookmarks that you made, by clicking on those 3 vertical dots and selecting "bookmarks" from the list. The most recent ones will appear at the bottom of the list. You can also manage those bookmarks in various categories, but that's another lesson.
In the forum, it seems that you should use "Watch" to watch a thread, but "Bookmark" individual posts (using the little icon beside the post number, that looks remotely like a paper bookmark) to locate specific posts for future reference. You can then label (for convenient filtering, if you have a lot) and describe them. I just created a couple to test it.
[Edited to add: Using the forum bookmarks, if you want to bookmark a thread, just bookmark the first post in the thread. You can bookmark the thread AND also specific posts within it.]
So, forum bookmarks seem a bit risky. Maybe make use of your browser bookmarks to identify threads of interest. (The bookmarked URL takes you to a full thread on the forum, not to a post.) The advantage of the forum bookmarks is that it locates a specific post. If it is important, you could use both, with the browser bookmark in case the forum ones are lost, and you would be able to recreate them by scrolling through the bookmarked thread to find a post of interest.