When you add information to Zotero from the AMBS (or another) library website, Zotero often converts the Library of Congress subject headings into tags. You can also add your own tags as an additional way to organize your library.
For example, suppose an AMBS student, let's call them Titus, has been adding all sorts of biblical and theogical materials to Zotero. In Titus's final year at AMBS, they are asked to write a paper on Nahum. Titus can go to their Zotero library, click the "Bible.Nahum." tag, and see all the references in their Zotero library that share that tag.
Or perhaps Titus is tasked with reflecting on their work at AMBS. As Titus has been researching for various projects, they have found a lot of resources that they think are worth reading, but that the ultimately disagree with. When they've found these, they tag it in Zotero with "WRBID" (worth reading, but I disagree). Titus can now search for all resources tagged "WRBID" and do whatever it is that Titus wants to do with that information (maybe find the common thread that prompts their disagreement).
In addition to bibliographic information, you might want to keep track of all the notes and quotes you're noting and quoting as you do your reading. In Zotero, you can keep track of notes in two ways.
Standalone notes
Think of standalone notes as odd thoughts or miscellaneous things you want to keep track of. Many people use Microsoft OneNote, Evernote, Notion, Joplin, Google Keep, or Google Jamboard to do something similar, and all of those tools are more full-powered than Zotero's standalone note feature.
The advantage of using Zotero is that all your notes are incorporated into the same place that you keep track of your bibliographic information, and can be tagged the same way as other resources. So as you start building your Zotero library and find yourself searching for resources about a topic, your notes can be returned in the search results.
Resource-specific notes
Resource-specific notes are notes tied to a resource. Use these to summarize books and articles, to store relevant quotes, to note key arguments (both those the text makes and those you have with the text!), and to add other comments and thoughts about the resource in question.
This is an excellent way to keep track of notes from a presentation at a conference, for example.
Though Zotero does not contain the full text of eBooks, you can add PDFs to Zotero. In fact, Zotero may do this for you automatically. To check:
What do those options mean?
"Automatically attach associated PDFs" means that when you add a journal article to your library, Zotero will try to add a PDF of the article itself.
"Automatically retrieve metadata for PDFs" means that when you manually add a PDF to Zotero (for example, an article an instructor posted on Moodle), Zotero will try to extract bibliographic data from that PDF.
Over time, as you use Zotero, it begins to be a place you can go to re-read articles you've already encountered. You can also add your own work so that you can cite yourself later.
