Offset is not the same as a timezone. Offsets change throughout the year in the same geographic areas (or don't or both do and don't)

No, timezones don’t change but they are swapped out by countries, offsets and timezones are an n:1 mapping. Multiple timezones represent the same offset, but those offsets are immutable.

CEST will always be +2 UTC (unless something really changes politically).

DST just marks that Sweden changes from CEST to CET from October to May.

At that point, just give me

  2025-09-07T11:30:31.304[Europe/Riga]

the machine can figure out the current offset and GMT itself because in any given time in the year I have no idea without looking it up.

That’s wrong or at least not normally how the term “time zone” is used. We would typically say that Sweden is in one time zone whose offset changes twice a year.

so CEST is what?

An offset from UTC.

Wikipedia calls it "the standard clock time observed during the period of summer daylight-saving in those European countries which observe Central European Time (CET; UTC+01:00) during the other part of the year."

Note the word "zone" does not appear in the above.

Based on a quick skim, CEST is a "time" not a "time zone".

https://en.wikipedia.org/wiki/Central_European_Summer_Time

https://en.wikipedia.org/wiki/Time_zone

https://en.wikipedia.org/wiki/Standard_time

There is the timezone "Amsterdam". That isn't a fixed offset.