The various problems across the devices are well documented online and vary wildly.
That said, I recommend you start over from scratch. There are some different approaches to this, btw. Here's how I would proceed:
1) Manually export/backup the songs/media you own and haven't purchased from iTunes. Put them on a separate PC or flash drive, apart from your Dell.
2) Backup all critical info on your other devices. Note all passwords/etc. for reference as needed.
3) Create iCloud backups of your devices.
4) Create local backup of your devices on the PC.
5) Delete iTunes (and all other Apple software) on your Dell.
6) Completely wipe (=erase all content from) your iPad and iPhone.
7) Make sure PC is clean (e.g. thorough anti-virus, malware check, registry free of errors)
Reboot PC before re-installing iTunes/etc.
9) Re-configure your iPad and iPhone. Do *not* download any media yet. (Ugh, I know.)
10) Re-install iTunes, other associated Apple software for Windows 10.
11) Reboot PC again(!)
12) Import your presumably 'clean' media content from backup on flash drive.
13) Download the songs you have already purchased into iTunes.
14) Sync library with your respective Apple devices.
15) Test some of the music out you have downloaded on each device (make sure not corrupt).
16) Reboot everything and test again.
17 Keep up with iTunes/iOS/Windows updates.
I have an iPhone 7 Plus 128 gb and an iPad 2 64 gb, and I control them with my Dell XPS8500 Windows 10 Pro computer. I currently have about 3,000 songs in the music library, and I've created some playlists Here's the problem:
I can go to iTunes, purchase a song, add the song to a playlist, and everything is OK on the computer. When I connect my iPad or iPhone, the song will download to the devices memory but will not appear in the playlist no matter how many times I sync the device. The only way to make the changes in the devices is to do a complete restore, and I've done that twice. It's no fun. Both times I wound up locked out and had to make a 70 mile round trip to my closest Apple Store
In addition, I just tried again with my iPhone and the same thing happened but when the sync finished, iTunes also attempted to sync my iPad, and it wasn't even connected to the computer.
TWICE, I went to Apple Support online for help with this and both times they made it worse. First time, the tech on the other end, who was screen sharing with me, had me go to to Music on the device and check the box for syncing playlists. I did that and it removed the 2,500 or so songs I had that were not in playlists.
I had to re-add them and when I did, more than a third didn't download properly (dotted circles to the left of the title). Back to Apple Support and the second tech said she'd have to connect me with someone up the line, then disconnected me completely. (Is it normal to call Apple Support for help and hearing a baby crying in the background?) I wound up uninstalling and reinstalling iTunes on my computer to try to fix that.
For that matter, I have uninstalled/reinstalled iTunes several times, following the prescribed order of program removal but I have never removed the info files, but it's not helping me.
Can anyone advise on this? is a setting off? Any clues?