Zotero is of course a database for bibliographic references, but you can also create standalone notes for ideas or concepts.
To create a note, menu "File" -> New note or 
button.
Notes can be tagged, linked to references (or to other notes) via the Related tab (the Related and Tags functions are located at the bottom of the note), and placed in collections. Zotero's search features make them easy to find.
"I take notes in many different places, but I ultimately store them in Zotero with the relevant references. The benefit is that you can use the search and tagging features for notes too. For instance, I use tags to keep track of good quotes I might want to use, and I copy and paste core arguments into notes so that I don’t have to go through a whole article again." (Mark Dingemanse, Radboud University Nijmegen)
"The real value in using Zotero is building a searchable, sortable, annotated, and organized database of literature. That's what will really pay off 10 years down the line." (Sebastian Karcher).
Here are examples of how some people use it:
Zotero Better Notes helps you to keep your notes organised. You can create main notes, with parts and sub-parts, and link your notes there. There is a button 
in the Note Editor and in the built-in PDF reader to add the link directly to the right place. Zotero Better Notes adds an extra workplace to your Zotero that opens in a new tab. Graphical visualisation options are available in the outline tab on the left. You can create customised note templates, export your notes to various tools, and sync them to Markdown notes editor like Obsidian.
