Revolut Senior Software Engineer - Java Developer interview questions

Company for some reason makes a strong accent on concurrency initially. I was asked to create a load balancer with the production like code. You won't be given any requirements initially but only to implement the register method. Tried to clarify all the requirements ( target groups, healthchecks etc. ) From the interviwer answers I realized I shouldn't clarify requiments at this point. More requirements will be added added like capacity limitation and uniqueness check. why do you prefer register(String url) over register(URL url)? in what case the array list append won't be constant time? why it's not ready for prod ( concurrency ) what problems do you see? what's the simplest was to fix it? Didn't receive any feedback so it's hard to tell what was expected.