On the left side of Files, there are three blocks (top,middle,bottom).
The middle block are the bookmarks, you can move folders there and move them over “new bookmark link” to create a new bookmark with the folder.
Or simply open the folder or / move mouse to the middle block and press Ctrl & d to get a new bookmark.
To make it permanent type-able use: gsettings set org.gnome.nautilus.preferences always-use-location-entry true