Install Playstore on Android 9 image

[boplinger]

How do I do any of this???  I am brand new to Android development.  This is LITERALLY the first thing I am attempting.
1.  How and where do I enter these commands?
2.  Do I need to make a physical connection between my PC and the Rock64?  If so, what type of cable do I need.
3.  What apps or programs do I need?
4.  I'm not sure what other questions I even need to ask, but an hour of googling has taken me round in circles, so I back here asking the "dumb" questions.
Thanks.

For a Rock64 you will need a USB-a to USB-A cable, to connect your ROKC64 to you computer. On that computer you will need to install adb tools. (The package name can vary based on the OS you're using on your computer.) On the ROCK64 Android install, you will need to go to Settings > Developer Options and enable USB Debugging.

Once you've done those things, connect your computer and ROCK64, using the USB-A cable, you should be setup to follow the instructions given, from your computer's terminal. (You will likely need to run 'adb devices' or 'adb start-server' to start adb and let your computer see the ROCK64.)

Thanks for these instructions. 
I am running into the following issue:
I have a USB A/USB C cable with a USB C to USB A adapter plugged into an USB A extension cable from my PC to the Rock64.  I have installed and am running "Platform Tools".  I launch a PowerShell terminal and type "./adb devices", hit Enter, and get the following back: "List of devices attached", but nothing is listed.
Any ideas???
Thanks!