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, which can then be annotated inside EndNote.
You can easily annotate PDFs in EndNote. Double click any reference that has an attached PDF, and then click on the PDF. From there, underline, highlight, or add notes.
Sometimes articles may be retracted, corrected, or otherwise flagged after publication. Like the citation manager Zotero, EndNote 20 has 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 That means you'll be notified if an article in your library has been retracted.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).