Resource Linking replaces the Resource Manager that was in earlier versions. It does much the same job and provides a means of accessing various files such as images that are maintained by another part of the author's organization and are liable to change.
When you upgrade a project,
any resources being used will be brought into your new project and be
linked. You will need to create links as below to any folders and files
that were not in use at the time of upgrade.
Simply right click in the Contents Panel and click Link > Link File
or Link Folder and browse to what you want to see in Adobe RoboHelp 2019.
In this image a folder has been added.

Just drag any image into any topic as required.
Whilst the arrow remains green, you know what you are using is in sync with the source. If it appears red, then the source has changed but your topic will not automatically update. It may be the source has been changed in preparation for say a new corporate image but for now you want to leave the old version, that's OK. When you are ready to use the updated image, click the menu icon that displays while hovering over an image and click Sync.
After you have linked a folder,
you may see many resources that you know you will never want. You can
delete those from here. Hover over the image and click Delete in the menu.
What you see in Adobe RoboHelp 2019 is a snapshot
of the files at the time the folder was added to Adobe RoboHelp.
New files added to the source folder will not be shown but any you need
can be added.
Do not rename any folder or
file in either the source location or Adobe RoboHelp 2019
as that will break the linking.