Are terror attacks on American soil mostly done by non-Americans?

NO.

According to the Center for disease control and prevention,

From 2001-2013

406,496 people died by firearms by fellow Americans

From 2001-2013 terror attacks done by non Americans have taken 3,380 lives

Lives lost during 9/11 : about 3,000

US killed at least 1.3 million as result of 9/11