There are literally hundreds of endnotes for the Revelation 16 chapters. Rather than try to fit them all here, they are organized according to the same chapter divisions as the text.
Revelation 16:1-2 Endnotes (The First Bowl)
Revelation 16:3 Endnotes (The Second Bowl)
Revelation 16:4-7 Endnotes (The Third Bowl)
Revelation 16:8-9 Endnotes (The Fourth Bowl)
Revelation 16:10-11 Endnotes (The Fifth Bowl)
Revelation 16:12 Endnotes (The Sixth Bowl)
Revelation 16:13-16 Endnotes (The Third Interval)
Revelation 16:17-21 Endnotes (The Seventh Bowl)