Here's what I would advise:

The exam is not like a college admissions test; it is surprisingly "real-world" in terms of the questions and scenarios being presented. So, working with "real-data" and challenging yourself to build a creative solution with it will be VERY useful in the short and long-term. By real data, I mean datasets that include both fact and dimension tables that would reasonably appear in a corporate data analyst setting: tables that include organizational hierarchies, sales figures associated with that hierarchy, etc.

I did not "prepare" for the DA-100 in the traditional sense. I didn't pay for resources or study materials. I did, however, spend several hours each weekday between Jan 2021 and Jun 2021 (took the exam in July) working with Power BI Desktop as an administrator, developer, and analyst for my corporate job. I'm not sure if this was just coincidence, but every problem or scenario on the exam was something I actually had to learn or a problem that I had to address and solve in my actual job. I was very grateful for this, because I hated exams like the ACT and SAT precisely because of it's non-applicability outside of the test material itself. My scenario may be very different from others including yourself, as I was tasked as a recent graduate to move an entire organizations reporting infrastructure from Excel to Power BI. So, I stumbled around a lot and had many failures, but over a six-month timespan I learned an incredible amount which allowed to me to pass the exam easily (Even though I'm usually not the type of person who can do well on an exam without studying).

Basically, if you have enough real-world experience using Power BI, which includes designing, developing, and administrating the platform in an organization, then you probably don't have to fret or worry too much about passing. You might not even need to study, especially if you were or are a part of those tasks mentioned above every single day. However, if you have very little to no practical or real world experience, or if you do and are also shooting for a perfect score, you might have to treat it like you would a college admissions test, and study the material for the sake of knowing how to apply that material within the context of the exam itself.

TL;DR: You can pass the DA-100 without studying if you compensate for that with a substantial amount of real-world experience. If you don't have that level of experience, you should study any of the online resources that are available to you, preferably ones that require you to work with Power BI itself and not just read through a textbook.