"If two sinusoids with much different periods are combined to form a periodic function with a varying sinusoidal axis, which operation was used?"

Response:

Multiplication.

"If two sinusoids with much different periods are combined to form a periodic function with a varying amplitude, which operation was used?"

Response:

After testing both addition and multiplication out, it seems as though both have varying amplitudes, but multiplication seems to be the most compelling.

EDITI figured it out