I recommend opening the bootstrap java project in an IDE and navigating around to answer questions like this. The simple answer is adb starts uiautomator which runs the bootstrap test case. The test case creates a socket server and listens for commands. AndroidCommandExecutor is responsible for running the commands.
I am novice to Java programming in fact Appium also. The three links you provided redirects to source code of your javascript/java files. For me at the current situation to understand this, it takes months time.
So can you/anyone from Appium team, please share a presentation slide/architecture diagram if you have.
Yes, that looks correct. One thing to note is because we’re using the JSON Wire Protocol, appium’s client libraries are all built upon the upstream Selenium client libraries.