Hi deborahfrombemidji
welcome to this Apple (user-to-user) Community!
Re: "I want to add more music to my iPod 6th generation. "
Am no expert on the iPod, however your post deserves some response.
Fortunately, Apple support still has articles and documentation available online.
You might want to check which iOS version is currently installed on your iPod 6th generation
Apple support's article for 6th gen iPod Your iPod touch - Apple Support is for iOS 12.3
and says:
"You can update to the latest iOS software if your model supports it."
and
"To see the model and (current) iOS software version of your iPod touch, go to Settings > General > About.
See Get information about your iPod touch"
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Re: " Looking for the music on the app, I can't find it. How can I find the old lists and then add new music."
You may have a "Library" jn the Music app, with your music included there.
Apple Support's "Music" articles (links below) for iPod with iOS 12.3
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Re: "(Also was able to get my old Nano to work as long as it's attached to a dock.
Would love to have the music from my iPod on it and the music on the Nano on my iPod.
I'm assuming the Nano is no longer supported.)"
You may want to check out which Nano model you have ... you can scroll down to find it in the article below
Identify your iPod model - Apple Support
Eg: (You may (or may not) find it's user guide online by typing in the particular Nano model )
All the best :-)