Nautilus is like MacOS Finder.

Edit these two files to comment out the standard Nautilus sidebar items that you don't want:

~/.config/user-dirs.dirs
/etc/xdg/user-dirs.defaults (requires sudo)

Restart Nautilus after editing the first file. Upon login, the first file may be overwritten by the second file, hence the need to edit the second file.

To restart Nautilus:

killall nautilus

That should kill any open windows and you can open a new one.

Now the items you commented out in the config file will be in the second "Bookmarks" section of the sidebar, and you can right-click and remove them.