How to collect a list of file URLs using Rules and send a single email containing that list?
Concatenating text in Rules was difficult when I did it. What I did was to keep two variables: a text array with a starting element value (such as "This is a digest:...") and a text value.

For each iteration of a loop over items to concatenate into a digest, etc. I set the text value to be the first item in the array ([array:0]) and then set the [array:0] value to be a concatenation of the text value and the new value from the loop. When the loop finishes, you know the [array:0] has the digest content.

I do not know why this couldn't work in a simpler way by concatenating the text value each loop directly or using two text values instead of an array. When I tried either of the other ways it only kept the last item or first item of what was being concatenated.