Update existing array items from another array

I'm trying to update an existing array's items with results from a separate array, matching based on a value match between the two arrays.

The first screenshot (premergedJSON) is the data structure prior to the merge, and the second screenshot (PostMergedJSON) is what I'd like the data to be structured as after the merge.

The key that I'm trying to match on in the first array is 'key' and the key from the second array to match on is 'field_id'. They both have the same value, and therefore I want the content from each matched array item to contain the contents from both arrays.

Thanks for any guidance on how to achieve this.
[premergedJSON.png][PostMergedJSON.png]