The feature called "bookmarks" was recently completely overhauled by Stack Exchange and it is now called saves:

 * [Bookmarks have evolved into Saves](https://meta.stackexchange.com/q/382019)
 * [Recent feature changes to Stack Exchange](https://meta.stackexchange.com/q/59445#375078) - October 2022
 * Help Center: [How do I use saves?](https://mathoverflow.net/help/saves)

One of the changes is that now you can organize your saves to several lists and give them any names you wish. And you can also add a private note to each of the saved posts.

You can find some other useful links in the corresponding tag-info both [on this meta](https://meta.mathoverflow.net/tags/saves/info) and [on Meta Stack Exchange](https://meta.stackexchange.com/tags/saves/info).