Started the process with an application/resume submission, followed by a Virtual Job Tryout.
After that, I was given a CodeSignal task where I had to answer 13 questions within 2 hours, based on a large spreadsheet with numerous data tables.
I was invited to a Power Day after completing the CodeSignal assessment. Power Day consists of three 1-hour-long interviews; two of them are business case interviews and one is a product interview. Business case interviews involve a hypothetical business in a particular scenario, where the main elements of the interview are creating a framework to increase profitability, calculating important metrics based on provided numerical data, and suggesting recommendations based on your quantitative analysis. The product interview requires you to have thorough knowledge of a digital app or social media platform, where you will describe its business model (what drives costs & revenues) and make suggestions for the company to increase profitability.
C1 is the only employer I know of that does not have a behavioral interview component for its internship program (which is why I really liked the hiring process). It's all about your capabilities.