The extra supply is balanced by the extra demand only if exports are zero. I.e. demand for Apple devices won't increase by 10% if the US population grows by 10%. And of course making 10% more rarely requires 10% more workers - you need more on the assembly line, but not more to design the items.
It wouldn't. Say someone in the US buys something made in China - how does that increase demand for labor in the US? There's some marginal US labor required for the last part of the transport, but that's it.
Thee last part of the transport is not marginal in the real world. Increased local consumption drives transport, retail, marketing, etc. etc.
Real life economic modeling is not a simple supply/demand, widgets made/sold chart.