Unless you have selected a custom data directory in the Advanced pane of the Zotero preferences, your Zotero data is stored within the following OS-dependent directories:

macOS - /Users//Zotero
Windows 7 and higher - C:\Users\\Zotero
Windows XP/2000 - C:\Documents and Settings\\Zotero
Linux ~ /Zotero

Reference: https://www.zotero.org/support/frequently_asked_questions

Option A: Copy the data directory

The most reliable way to move your entire library to another computer is to copy the Zotero data folder from your first computer to your new computer. To locate your Zotero data, open the Zotero preferences, go to Advanced → Files and Folders, and click “Show Data Directory”. See here for the default locations of the data folder. Be sure to close Zotero on both machines before copying the Zotero files. If you've already opened Zotero on the new computer, there will already be a Zotero data directory with an empty database, and you should delete the whole data directory before copying the new directory to the same location.

Option B: Enable Zotero Sync

You can also sync your library between multiple computers via the Zotero servers. Make sure you use the same username on all computers. If syncing files, you'll need enough online storage space to fit all files in your library. Zotero will warn you if you hit your quota, in which case you may need to delete some files, add a storage plan, or transfer your library using Option A above.

Warning

Exporting and importing your library (for instance via Zotero RDF) is not a recommended option. None of the available export formats allow for a complete transfer of your library data, reimporting will break connections with any existing word processor documents, and if you use syncing later you will end up with duplicates of any imported items.

Reference: https://www.zotero.org/support/frequently_asked_questions

The best way to access your Zotero library from multiple computers is to use Zotero syncing. Zotero syncing will automatically sync your library data using the Zotero servers. Attachment files can be synced using the Zotero servers or using a third-party WebDAV servers. You absolutely should not store your Zotero database in a cloud storage folder (e.g., Dropbox, Google Drive, OneDrive), which will lead to data corruption. You can, however, configure Zotero to sync your attachment files using these services while using Zotero syncing to sync your library data. If, for whatever reason, you cannot use Zotero's built-in sync features, you can also store your Zotero data directory on an external hard drive and use this to move your Zotero data between computers. You can set your Zotero installation on each computer to point to the external drive in the Advanced pane of Zotero preferences. Another option is to run a copy of Zotero directly from a portable drive.

Reference: https://www.zotero.org/support/frequently_asked_questions

A book chapter from an edited volume is entered as a “Book Section”. Click the green new item button and select “Book Section”. You will now see both a “Title” field for the chapter title and a separate “Book Title” field. To add an editor click the + sign on the author line in the right column. This will create an additional Author line. If you click the “Author” label you will be able to change it to an editor.

Reference: https://www.zotero.org/support/frequently_asked_questions

To see all the collections an item is in, select the item and then hold down the Option key (macOS), Control key (Windows), or Alt key (Linux). This will highlight all collections that contain the selected item.

Reference: https://www.zotero.org/support/frequently_asked_questions

Citations inserted using a reference manager Zotero can read existing citations created by the Zotero and Mendeley word processor plugins, allowing you to continue using those citations in the same document even if the items don't exist in your Zotero library. Simply click Add/Edit Citation, search for an existing citation, and select it from the Cited section of the search results. (This applies to the default citation dialog only, not the “classic” dialog.)

If a document contains Zotero or Mendeley citations not in your library and you need to make changes to the metadata or include them in other documents, you'll need to extract the citations into your library. For Word .docx documents, you can use Reference Extractor. Note that to continue using the same document, you'll want to replace all instances of the original citation with the new item from your library, being sure to select from the library section of the citation dialog's search results rather than the Cited section.

If you still have the references in a reference manager, you can export them from that program in a format such as RIS or BibTeX and then import that file into Zotero. You'll need to replace all existing citations in any document for which you want Zotero to generate a correct bibliography.

Citations inserted using Microsoft Word's built-in citation feature

You can follow these steps to format the bibliography as BibTeX, which Zotero can import:

Download this Word bibliography stylesheet. Save the stylesheet to Word's bibliography styles folder: Word 2016/2019/Office 365 for Windows: C:\Users\\AppData\Roaming\Microsoft\Bibliography\Style Word 2010 for Windows: C:\Program Files\Microsoft Office\\Bibliography\Style or C:\Program Files (x86)\Microsoft Office\\Bibliography\Style Mac: Go to the Applications folder. Right-click on Microsoft Word and choose “Show Package Contents”. Navigate to: Content/Resources/Style In Word, change your bibliography style to “BibTeX export” and copy the bibliography to the clipboard. Use Zotero's Import from Clipboard function. To continue using the same document, you'll want to replace all instances of the original citations so that Zotero can generate a correct bibliography.

Plain-text citations and bibliographies

If the references have ISBNs, DOIs, or PubMed IDs, you can use the Add Item by Identifier function in Zotero to quickly add these items to your Zotero library.

If you have many references, you can use AnyStyle, ​an online bibliography ​parser written by a Zotero developer. Export parsed citations as BibTeX or CSL-JSON and import them into Zotero.

Otherwise, your best option is to find the items online in a repository that Zotero supports, or, as a last resort, manually enter the references.

In all cases, you'll need to replace all existing citations in any document for which you want Zotero to generate a correct bibliography

Reference: https://www.zotero.org/support/frequently_asked_questions