Reformat one json file by example

Time after time I need to merge two large jsons files, or more precisely add a json fragment to another file.

The too pieces are often written by different people and have different formats. Cut and paste results in ugly code with different spacing.

How I can reformat the fragment in the same style the first file is formatted. I do not know what IDE, formatter, or style guide was used. I use IntelliJIdea but open to try any other tool.

I know that I can use automated tool to reformat the whole document into whatever is supported.

PS. I cross posted into AI but I imagine for such a simple language the problem might be addressed by some simpler methods