Geometry Dash custom songs download location
You'll need root access to find the folder.

Steps:


Root your phone
Install an application like ES File Explorer that has the root explorer function.
Go to /data/data/com.robtopx.geometryjump/files
Paste song, name of song must be song ID (Newgrounds)


Note: If you don't see the folders after /data/data, then either the file explorer doesn't support root explorer or your device is not rooted.