Either approach can work. Naming all branches first gives the interviewer an overview, while explaining one branch at a time can sound more natural.

A useful compromise is to preview the two or three main branches in one sentence, then explain each briefly. That preserves the top-down structure without becoming robotic.

Keep the explanation short enough that the interviewer can still remember the overall map. If useful, this guide on case interview structure and flow gives more detail.

I preview all buckets then explain. keeps it top-down.

one by one sounds more natural for me though.

Whatever you do, don't spend 3 mins on structure intro.

I forget bucket 3 if I over-explain bucket 1 lol.

Short preview + detail is probably safest.