Basically, they use another process that glues them all together into the GUI.
Google Chrome creates three different types of processes: browser, renderers, and plug-ins.
Browser: There’s only one browser process, which manages the tabs, windows, and “chrome” of the browser. This process also handles all interactions with the disk, network, user input, and display, but it makes no attempt to parse or render any content from the web.
Renderers: The browser process creates many renderer processes, each responsible for rendering web pages. The renderer processes contain all the complex logic for handling HTML, JavaScript, CSS, images, and so on. Chrome achieves this using the open source WebKit rendering engine, which is also used by Apple’s Safari web browser. Each renderer process is run in a sandbox, which means it has almost no direct access to the disk, network, or display. All interactions with web apps, including user input events and screen painting, must go through the browser process. This lets the browser process monitor the renderers for suspicious activity, killing them if it suspects an exploit has occurred.
Plug-ins: The browser process also creates one process for each type of plug-in that is in use, such as Flash, Quicktime, or Adobe Reader. These processes just contain the plug-ins themselves, along with some glue code to let them interact with the browser and renderers.
Source: Chromium Blog: Multi-process Architecture