When we install Appium on our PC, it also installs a server that exposes the REST API. It accepts commands and connection requests from the client and executes them on iOS or Android devices. It responds to HTTP requests with HTTP responses. It RUNS the user interface of the app using a mobile test automation framework to perform requests. As an example -

UIAutomator is used for Android API 16 or higher, while Selendroid is used for Android API 15 or below. Apple Instruments is used for iOS.

Working of Appium on Android:

Appium sends the command to a UIAutomator script running on the device on Android. UIAutomator is an Android native UI automation framework that allows you to run Junit test cases straight from the command line on the device. Despite the fact that it is written in Java, Appium can be run from any WebDriver enabled language.

Android makes use of bootstrap.jar, a TCP server. It's used to deliver test commands to an Android device, which UIAutomator then executes.

In the above IMAGE, we can clearly SEE the architecture of Appium used for running on Android devices.

Working of Appium on iOS:

As Android uses UIAutomator, iOS uses UIAutomation. Similar to the Android, Appium PROXIES the command to a UIAutomation test case running on the Mac instruments ENVIRONMENT. Apple provides this application "instrument" that performs various activities like building, profiling, and controlling iOS apps. On the other hand, it also has an automation component where you can write commands in JavaScript. It uses UIAutomation API to interact with Application UI. Appium uses the same libraries to automate iOS Apps.

In the above image, we can clearly see the architecture of Appium used for running on iOS devices.