STV is the worst visa imaginable. Yes, insurance for the entire trip. Yes, must book abd pay for your entire trip in advance. No flexibility at all. You cannot exchange or extend.
if they didn't ask for proof of insurance when you applied... Then why would you worry. Though each immigration location seems to do whatever they choose, if your location didn't bring up the subject, why would you? Just take the extention they give you
If you apply for retirement before you arrive, O-A visa, you must buy expensive health insurance for a year plus covid insurance. If you apply for retirement after you arrive, theres currently no health insurance requirement