The following paragraph is very confusing
We assume that there is a specific second in the day when all the requests of all the users arrive at the service simultaneously. We use it to get the capacity estimation for a peak load. To do better, we will need request and response distributions, which might be available at the prototyping level. We might assume that distributions follow a particular type of distribution, for example, the Poisson distribution. By using DAU as a proxy for peak load for a specific second, we have avoided difficulties finding the arrival rates of requests and responses.
First of all, you pointed to an unexplained term → request-response distributions. What is a request-response distribution? Then you pointed to the Poisson distribution. You are assuming that the reader understands and that is completely wrong. Also what is the prototyping level?
Please do not point to unexplained methodologies because this confuses the reader.
Course: Grokking Modern System Design Interview for Engineers & Managers - Learn Interactively
Lesson: Examples of Resource Estimation - Grokking Modern System Design Interview for Engineers & Managers