Adobe exchange is sort of a marketplaces for extensions, plugins etc for Adobe applications. Think of it like appstore on an Apple device or Google Play Store on an Android device
Click on the following URL
https://exchange.adobe.com/creativecloud.illustrator.html#product
Search the extension/plugin that you need and then you can install it right from the extension/plugin details page. Make sure you are logged in using your Adobe ID
You can access the same URL from within Illustrator application as well. Click on Window>Find Extension on Exchange... and it will open the webpage i mentioned in the browser.
-Manan