It's simple, but there are several preconditions.
First of all, you have a USB flash drive that can be plugged into the charging port of your mobile phone, commonly known as the adapter.
Secondly, your mobile phone must support otg function. Of course, if not, it is ok. You can download the ES file browser when inserting the USB flash drive. This APP can automatically identify USB flash drives, which is quite powerful.
Third, you must have a music player, where you can search for songs to download. You can set the download path in these apps and change it to a USB flash drive.
Then you can search and download songs.
If none of these are available, download them from your computer and copy them to a USB flash drive.