Hi MysteryG45,
I see that you have some questions about syncing a random selection of songs from your iTunes library. The feature you are asking about is called Autofill, and more information about it can be found here:
If you don’t want to always hear the same music on your device, you can have iTunes fill it with a random selection of songs by using Autofill.
To use Autofill, you must turn off automatic syncing for music and videos (see instructions above).
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Connect your device to your computer and click the device button near the top left of the iTunes window.
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Click On My Device.
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Select Music in the sidebar on the left.
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Choose which songs you want to sync from the Autofill pop-up menu at the bottom of the Music pane.
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To adjust Autofill settings, click Settings.
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When you’re done, click OK.
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Click Autofill, then click Sync.
Each time you use Autofill, iTunes adds different songs to your device.
Sync iPod, iPhone, or iPad - iTunes Help
https://help.apple.com/itunes/mac/12/index.html?localePath=en.lproj#/itns5ecc4d9 8
Thanks for using the Apple Support Communities!