There are 4 ways to add references to your EndNote library:
You can organize your EndNote library using groups. To create a group, go to Groups --> Create Group. Name it, and then drag and drop references into it.
There are other types of groups:
You can also create a Group Set, and file groups themselves by dragging and dropping
When you add references, you're just getting the citation information. However, you can also attach PDFs and then annotate them inside EndNote.
You can annotate PDFs in EndNote. Highlight any reference that has an attached PDF, and then click on PDF in the right pane. From there, click the icon that is a box with an arrow coming out of it. (If you hover over the icon, it says 'Open the reference in a new window.' Once your article pops up, click the text bubble icon to access highlighting and underlining tools.
Sometimes articles may be retracted, corrected, or otherwise flagged after publication. Like the citation manager Zotero, EndNote 20 and 21 have an integration with Retraction Watch. That means you'll be notified if an article in your library has been retracted. In EndNote you will see a folder on the left side (under 'All References') that says Retractions. Any retracted articles will be put there (and will stay in all references as well). EndNote will also notify you if you cite a retracted article using Cite While You Write.
Note that to see retracted articles you will need to have one library synced with EndNote Online; you will then see retractions in all of your libraries, regardless of their sync status.