The hiring manager or interviewer took them through a system and basically ask them pointed questions on algorithms and data structures.
What they enjoyed was that it was both in-depth, open, and not something one could just cheat/memorize their way through.
What were your most memorable and thorough tech interviews?
One of the best was given a PR to review with the context, "this was from a junior engineer and you're a new senior, how would you review it?"
I'm leery of using real pull requests though: Some engineers would assume it's a free labor in the guise of an interview.
IIRC (this was about 13 years ago so a little fuzzy) it was a simple SMTP server and the vulnerability was a stack overflow that could be triggered in the RCTP request so you needed to send some valid input first and get a few steps into the state machine.
For that kind of work, it doesn’t get much more real or more fun.
Unfortunately I was not a cultural fit as I was an order of magnitude more knowledgeable than my managers. They were not pleased that the exercise sample code I produced was a quarter the size of what they expected.
I worked for a number of Canadian banks and some half took great pride in their mediocrity.