Your experience. At the depth you choose.
A 20 minute voice interview about your own work. Pick how wide or deep it goes, add the job description you are preparing for if you have one, and come back as often as you like: each session picks up what the earlier ones did not cover.
You cut p95 latency by 40%. What was the bottleneck?
How did you know it was the cache, and not the database?
If you rebuilt it today, what would you measure first?