How to customize rss feed

How to customize rss feeds? Nowadays they look like this: https://www.duefiliderba.com/en/feed

I would like to change the last sentence, which is put automatically, in English, shorten the summary and add a read more link.

Where and how to change those options?

I searched in the wp-includes and there are like 10 different files.. Really don’t know where to look and what to change.

Thank you for your help!