The full process requires 70-80 hours from the candidate: there are two take-home assignments, each requiring 25-30 hours to do well enough to clear their bar. These are followed by 30-minute discussions of the task where your thinking will be challenged and additional case-style questions will be asked. Interviewers are often late, but they will also go over if you have questions. They also record your interview so the whole team can watch and weigh in on whether or not to move you forward.
After these rounds, there can either be a third take-home case (again, 20+ hrs of work) or an Amazon-style behavioral interview (45 min + the prep you do for it), and a conversation with one of the two CEOs. (When you speak to the CEO, they don't have any clue who you are, what role you are interviewing for, or where you are in the process, FYI - so don't let that throw you.)
The cases are fun, but the overall process is very challenging due to the time required. (It's also a reasonable hypothesis that they are simply crowdsourcing free ideas and work for their platform through these cases, as others have suggested.) If you have a full-time job or important life responsibilities while applying, it is nearly impossible to clear their bar at each stage simply due to the amount of time required. CH erroneously believes it has to do with how "smart" and "capable" applicants are, without recognizing the people who are smartest and most capable are likely already in jobs that would prevent them from putting 80 hours into a single job application.
Note that you also won't have a single point-of-contact, and it can be challenging to get questions answered between interviews. They also take about a week to review each take home exercise and come back to you with an invitation to do the live debrief, which can then take another week to get scheduled. Overall, the process took ~3 months to go through all 4 rounds.