Above用来表示一事物高于另一个，且它们之间有空间；而On top of用来表示一事物停留在另一个事物上。
The lamp is above my head.
The cap is on top of my head.
*Ming Lee is studying on top of me on the second floor./ Ming Lee is studying above me on the second floor.
*Do not sit above the table./ Do not sit on top of the table.
Reference
Teo, Alvin. 2019. Confusing Grammar Explained. Singapore: Marshall Cavendish Education.
