What are Site Links?
Site Links are a hybrid media technology that allows you to connect core sites to each other to share resources between them. You can use Site Links to connect premises core sites to each other or to connect a premises core site to a cloud core site.
For example, if you have a premises core site with an Edge that has an external trunk, and a cloud core site, you can use Site Links to connect the two core sites. When you do, your cloud core site and any cloud branch sites can then access that external trunk on the premise core site.
For more information, see the Enable and configure Site Links section in the Convert BYOC Premises to hybrid media article.