The process is very structured, and very long. It takes about a month to do the full loop, you get a lot of feedback at each stage and you are able to meet a lot of people, which is good. However, I think this takes something away from interviewing at Monzo, because you are not able to meet people you will be working with. Because of this, it's hard to make a call on whether you'll be happy joining the company, as it's a gamble what team you'll end up in.
The first round is a recruiter screen, pretty standard. The next round is a call to discuss a past project. The following stage is a technical, take home or pairing. The final stage is a two hour system design and behavioural. Overall the process is very intense.
The system design was not a typical system design interview. They are clearly looking for a specific solution with very specific technology choices. It's obviously solvable in many ways but they aren't interested in hearing anything that diverges from the approved solution (study their tech stack, Kafka and Cassandra). So it's less of a real system design interview and more of a guessing game. Core concepts of system design are glossed over and ignored here, such as queueing choices, database redundancy, caching, etc.
The behavioural interview is focused heavily around impact and products. Have examples of your best projects. It's a bit over the top, almost like you are being interviewed for an Engineering Director role. From what I understand everyone goes through this too, which seems overkill.
The interviewers were nice, but it's clear that the pressure in this environment is at the absolute maximum once inside. I don't believe Monzo are paying enough to justify this, they are looking for FAANG-level employees and paying quite poor salaries. They probably need to consider adding £50-75k on their more senior bands. I would not have accepted an offer based on comp and my experience in the process.