Just अंदर means 'inside' used as an adjective or adverb.
'----- के अंदर' is a postposition which means 'in -----'.
You can think of के अंदर as 'inside of'.
For example,
'दादा अंदर हैं - The grandfather is inside.
'दादा घर के अंदर है' - The grandfather is inside the house