If the emulator is not appear to be connected to the ADB when you Run/Debug your android project on Android Studio. You may need to adjust where the emulator looks for the ADB.
Android Studio Emualtor for Android uses a registry key to identify the base location of the Android SDK.

Run(Win+R) regedit and navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Android SDK Tools. Modify the Path variable to the path of your Android SDK. D:\Android\Sdk for example.

Notice: If you cannot find the registry key, just create one.

The regedit structor looks like below:

Reference: Troubleshoot the Visual Studio Emulator for Android