The music you purchase on Google Play is always available to you on their servers, "in the cloud". To listen to it on the Nexus 7, you would usually stream it to your device by using the Google Music app. You can also download songs for listening when offline, but you must use the Google Music app to listen. Other media player apps won't work, as the song files are specially encrypted. This also means that if you can actually find the download location on your N7 and transfer the music file to your laptop, you wouldn't be able to play it back.
To listen to your music from Google Play on your laptop, or to download MP3 versions onto your laptop for transferring to your MP3 player, use your browser and log in to your account at . Then click on the My Music link. You can listen by streaming songs through the browser interface, or you can perform some limited MP3 downloads to your laptop.
To upload music from your laptop, or to download without limits, download and install Google Music Manager onto your laptop. You can get it my clicking on the Upload Music link while you're in the My Music section. Here are the details:
I just stumbled onto the fact that the CD transferred music on my PC will not play on Nexus 7 by using the "Google Play Music" option, but WILL play by selecting the Dice Player "open with" option. I'm happy to see that Dice Player will play both my video clips and CD's I have stored in my Windows 7 Music Library. .