The Zotero Connector for Chrome, Firefox, and Safari talk to the HTTP server in the Zotero client.
Word Processor Integration API
Zotero word processor plugins are components implementing a small set of interfaces. Development guidelines for these interfaces are described for LibreOffice, Word for Windows, and Word for Mac.
The Google Docs plugin implements the HTTP citing protocol, by performing a sequence of HTTP requests in a transaction. This is the preferred way to provide citing integration with Zotero for document processors.
The LibreOffice plugin implements the word processor integration API by acting as a server for a simple TCP/IP protocol. If out-of-process communication mechanisms such as COM and AppleScript as well as HTTP messaging protocol is not available, the TCP/IP protocol can be used instead.