Bookmark passwords

Assuming you know the website of the bookmark, click Safari > Settings/Preferences > Passwords at the top. Unlock the pane, then simply look for the website of the bookmark.

Find saved passwords and passkeys on your Mac - Apple Support

If you never saved the password for the bookmarked page, it won't be in Keychain. Most websites have a 'forgot password' link somewhere around the login area, you could try that.