I would also read the documentation. In the given example, for example, you don't know if the desired fixed format "YYYY-MM-DD" might depend on some locale setting and only works because you happen to have the correct one in that test console.
What does that have to do with my comment?
The OP explicitly wrote
> prompt> use javascript to convert a unix timestamp to a date in 'YYYY-MM-DD' format using Temporal
The answer ends in `toPlainDate()` which returns an object with year, month and day properties. ie it does not output the requested format.
This is in addition to the issue that `fromEpochSeconds(timestamp)` really should probably be `fromEpochMilliseconds(timestamp * 1000)`