Just one African country South Africa that’s there. How do people from Nigeria subscribe to iTunes and Apple Music?

Both iTunes and Apple Music are supported in Nigeria. not sure where you saw that just South Africa ii supported.

See here: link-> Availability of Apple Media Services - Apple Support

Nigeria

• App Store
  • Apple Arcade
• Apple Music
• iTunes Store
  • Music
  • Ringtones & Tones
• Apple TV app
  • Synced videos only
• Apple Books
  • Public domain books only
• Apple Podcasts

Apple accepts most credit and debit cards in Nigeria.

If they are being declined, contact the bank or card issuer, and make sure they allow this.

Link-> Payment methods that you can use with your Apple ID - Apple Support