Zotero

Collections and subcollections in your Zotero library function like folders and subfolders, allowing you to organize items according to your preferences. You can arrange your collections and subcollections based on types, topics, or specific projects.

To create a new collection or subcollection, use the top menu by selecting "File" → "New Collection..." or click the new collection icon at the top of the "My Library" section in the left sidebar. 

When creating a new collection, ensure that "My Library" is selected under the "Create in:" dropdown menu. If you want to add a subcollection, choose an existing collection from the "Create in:" dropdown menu instead.

When adding items to your library, you can select any existing collections or subcollections to associate the new items with, ensuring they are organized according to your preferences.

For each items in your Zotero library, you can add tags and notes to help you organize and annotate your research materials, which is especially  useful when you have large collections of items in your library.

Tags allow you to categorize and label items using keywords or phrases to make it easier for you to find and filter them later. Zotero automatically adds some tags when you add items via web browser, but you can also add and manage your own custom tags.

Notes, on the other hand, allow you to add detailed annotations or comments to items in your library. These can include summaries, observations, or reminders related to the source, helping you keep track of important information and your thoughts associated with each item.