In our testing of external links in modules, it seems that only some destinations will display in the Canvas iframe area. If you configure the link with the "Open in a new tab" option unchecked, there is a chance the user will see a blank gray box instead of the content. See attachment. Clicking the title at the top of the box delivers you to the content using a new tab, but it's not really clear that's required. Most users will just stare at the gray blankness not knowing what to do.
However, it seems YouTube* videos will display in the iframe and there might be other web sites as well. Our university site does not display directly.
Given this uncertainty, what are other campuses telling their instructors about adding links to modules? Is the recommendation to always check the box for "Open in a new tab"?
* I suspect that if a YouTube video owner disables the ability to share their video via an iframe (what YT calls embedding), then the video will also fail to display in Canvas without the extra click.