Slack Senior Software Engineer/Tech Lead interview questions

Spoke with a recruiter, who gave me the basic requirements and discussions of the positions open. Began the process with a hiring manager screening, and this was focused on one thing I did in my career, and questions around that. It felt a little canned, and didn't feel like the conversation was organic, just checking the boxes for the interview. That went well, and led to a take home coding test. This was building a basic iOS app from a single API. it wasn't clear what they were looking for other than make it work and don't spend a ton of time on it, so I didn't. I should have spent more time on this project, as it ultimately was why I didn't get an offer. I had a tech interview which felt more like a screener once I completed the take home test. This was more centered around college level questions and also felt canned for the most part, though the interviewer was personable. I got no reply/information for about 10 days, then got a pretty standard rejection letter. I was told I did well on the tech interview, but the take home test was not up to their standards, but did not specify which standards were not met or what they were looking for. Overall I felt the experience was positive, but I wish I could have spoken to them a bit more of my experience, previous work, and value I've brought to the other companies. I feel this would have been more of a benefit than the somewhat vague coding challenge