1.2 what is a complex system? » Melanie Mitchell's Introduction to Complexity
SFI's Resident Faculty, Melanie Mitchell, leads a free Introduction to Complexity course here on this platform, which is required for all Undergraduate Complexity Researchers before arriving on campus. To get ready for your application, you may be interested in watching some of sections on YouTube (link below), but also please feel free to enroll in the course, which will allow you to submit assignments as well.
https://www.youtube.com/playlist?list=PLF0b3ThojznRyDQlitfUTzXEXwLNNE-mI
https://www.complexityexplorer.org/courses/144-introduction-to-complexity