How does it know whether my number is seconds or milliseconds?
By digit count, and it always tells you which it chose. A 10-digit value is seconds (covering years 2001–2286) and a 13-digit value is milliseconds. Guessing wrong is not a subtle error — it puts the answer in 1970 or the year 55000 — so the interpretation is reported rather than applied silently, and you can override it.
What is the ISO week number?
The ISO 8601 week, which is not the same as "the nth week of the year". ISO weeks start on Monday, and week 1 is the week containing the first Thursday of January — so 1 January can fall in week 52 of the previous year. Financial and manufacturing systems use it, and it is a common off-by-one source in reporting code.
Which timezone is used?
Both. UTC is shown because it is unambiguous, and your browser's local timezone is shown alongside it with the zone name so you can see the offset being applied. Unix timestamps themselves have no timezone — they are always seconds since the UTC epoch.