Here's the overlay 1 code for Jungle Bros, for example:
#include "huc.h"
main()
{
  set_font_pal(0);
  set_font_color(1,0);
  load_default_font();
  put_string("PRODUCED BY OR",9,10);
  put_string("UNDER LICENSE FROM",7,13);
  put_string("FROZEN  UTOPIA",9,16);
  ad_reset();
  ad_trans(2, 0x0, 4, 0x0);
  cd_execoverlay(3);
}
It displays a Genesis-like startup screen, loads the custom ADPCM data bank from overlay 2, then does a cd-execoverlay() with the third overlay, which is another program.