How to customize feed elements?

Hello, I’d like to customize my feed to have these elements:

source
– name : site name
– link : site URL
– updated_at : UTC date, last feed update
– articles: (array) of articles

For each Article:
– title : title article (max 80 chars)
– excerpt : short intro about the article (max 160 char)
– lang : lang article in standard ISO 15897 (POSIX Locales)
– category : article category (max 30 char)
– created_at : UTC date, last article update
– cover : cover picture (res. max: 1000×500 px)
– link : permalink, URL article

Info about the feed:
Content-Type: application/json;charset=utf-8
Max 50 articles, sort by desc

In which file can I set all these info to show the feed properly?
In 3.2 section here there is no indication of which file I have to change.

PS: the website for which I need support is http://www.hellogreen.it (in italian)
And the URL for the feed is at http://www.hellogreen.it/api/get_recent_posts/

Thanks

https://ww.wp.xz.cn/plugins/json-api/