git clone https://android.googlesource.com/platform/system/adb

There are three components in ADB: Client, AdbServer, Adb Daemon. Client and AdbServer always run on your PC, Adb Daemon (adbd) run on your Android device.

When you input adb connect <serial>, why connection state become device or offline. There are 1 + 2 * N threads in AdbServer, every connection has a atransport object, every transport has two threads.

ADB Daemon will do something to check auth permission before send CNXN Packet to AdbServer.

When you input adb -s <serial> <command>, command may be shell or others.