In October 2022, Stack Exchange completely overhauled the functionality of bookmarks:
The new functionality is called "saves" rather than bookmarks. Unlike bookmarks, you can now create several lists and name them in any way you want. (But these lists are private, visible only to you - bookmarks used to be publicly visible.)
You can search among your saves using in:saves. And you can add a private note to each of the saved posts.
You can find more information about saves in the announcement linked above. And also in the corresponding tag-info on Meta Stack Exchange and on this meta.