Understanding LED chain systems
None of the systems seems to use a dedicated LED driver

The Konstsmide does have a (probably 31V) AC to DC converter (power adapter) in the socket:

And the &quot;System Decor&quot; needs a &quot;Starttrafo&quot;.
And the System 24 probably also needs 24 V DC for operation so it also needs a power adapter.
So indeed these systems do not have a dedicated LED driver but that isn't always needed. These systems use low-voltage DC to power the LED strips. In the LED strips there will be a series resistor in series with each LED or parallel/serial combination of LEDs.
This is commonly used in nearly all LED strips.
Having individual drivers for each LED isn't needed (low voltage + resistor is good enough), more robust (if an LED breaks, the rest can still function as normal) and cheaper to make.

are the individual strands arrangeable both in series, in parallel and mixed

No, you can only connect strips in parallel. If you connect in series then the voltage will be too low to make the LEDs light up. The only thing you have to pay attention to is that you do not connect more LEDs than is supported by the Power adapter.

can the individual strands be shortened?

Yes as long as that is done &quot;properly&quot; so leaving no exposed wires, no moisture can get in etc. Since the LED sections are in parallel, a section can be removed without issue.