I know the idiom round the corner means very near. My question is, why round and corner? What is the connection between 'round' and 'corner'?
According to Etymologyonline (https://www.etymonline.com/):
late 13c., "place where streets or walls meet".''
In the idiom 'the corner' means the corner of the building or the streets which can be reached very easily and quickly on foot.
The idiom is used when you mean any place in the nearest neighborhood.
The idea of 'around the corner' as something very close to the speaker or writer is extended in another idiom - 'To be just around the corner'.
It's been used in the sense "about to happen" since 1905 (from: https://www.etymonline.com/).