"is it possible to trace back data to individuals from aggregations or any other way?"
It depends on what the aggregation function is and how many user inputs you collected before computing the aggregate. Strictly speaking you are asking a differential privacy question, but a typical use of something like Prio would be to compute an average, which is a very lossy function. If I tell you that among 10000 users it takes an average of 500ms to load a web page, you are not learning much about individuals (especially since I said nothing about the variance).
What Prio does is to ensure that nobody will see raw user data, while still allowing aggregate information to be computed and allowing the inputs to be validated. In other words, the fact that you are focusing on what the output will reveal is a sign that on some level you trust Prio to do exactly what it is designed to do as a mechanism for protecting user data.
It depends on what the aggregation function is and how many user inputs you collected before computing the aggregate. Strictly speaking you are asking a differential privacy question, but a typical use of something like Prio would be to compute an average, which is a very lossy function. If I tell you that among 10000 users it takes an average of 500ms to load a web page, you are not learning much about individuals (especially since I said nothing about the variance).
What Prio does is to ensure that nobody will see raw user data, while still allowing aggregate information to be computed and allowing the inputs to be validated. In other words, the fact that you are focusing on what the output will reveal is a sign that on some level you trust Prio to do exactly what it is designed to do as a mechanism for protecting user data.