Collection to string (key: value, key: value)

Hello everyone. I spent whole day to understand it but I still haven’t resolved.

I need from collection:
key: value
key: value

[
    {
        "key": "value",
        "key": "value"
    }
]

To make a string
key: value
key: value

[
    {
        "string": "key: value \n key: value \n"
    }
]

This should work without specific key and value, because I get different data to collection. I have always the same structure of collection, just different data and quantity of key: value

The last step on shreenshot is wrong. I suppose I need to fix only the last step