Creating subcircuits is not trivial and for new users is not intuitive. Should be fixed.
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Creating subcircuits is explained in the manual, so this issue is fixed. See document sources here: https://invent.kde.org/sdk/ktechlab/-/blob/f67acf039e990b2a7c23eb5d15278a2dcb4a4a0f/doc/en/circuits.docbook#L72