Start with the basics.
All custom sounds have be in the ringtone folder on the phone. You can add sounds to the folder with a ringtone app or by connecting the phone to a PC and adding sounds to the folder using windows explorer.
You can not use the phone to move sounds to this folder using an app like the ‘files’ app. They will not be recognized.
When you select sounds for notifications or ringtones these sounds will be in the available channel list as ‘custom’