La cuchara está detrás del vaso
And we use “detrás” with verbs as a Estar because they show a location (static).
We use “atrás” with verbs of motion, for example, to go, look to:
Ve a la parte de atrás de la casa (go behind the house)
Mira hacia atrás (Look at behind you)
La cuchara está encima del vaso
We use arriba (up) in the same way that “atrás”. And we use “Encima” with verbs as a Estar because show a location (static).
La cuchara está dentro del vaso
“En” is a preposition and you can traslate as a “in”. “Dentro” show you that something is “into” or “inside”.