Both can matter, but in different ways.

For megapixels, what matters is pixel size on a given sensor size. If you pack more megapixels onto the same sensor, each pixel is smaller and collects less light, so noise per pixel can increase. However, that noise is also finer-grained and may be less noticeable when the image is viewed or resized at the same output size.

For ISO, the common idea that “high ISO causes noise” is only partly true. ISO itself is amplification. The bigger reason high-ISO shots look noisier is usually that less light was captured in the first place, often because you used a faster shutter speed or smaller aperture. That reduces signal, so the signal-to-noise ratio is worse before amplification.

If you raise ISO while keeping shutter speed and aperture fixed, ISO alone is not the fundamental source of the extra noise; exposure and total light captured are the key factors.

So: noise is mainly about signal-to-noise ratio, sensor size/technology, and how much light you capture. Megapixels affect pixel-level noise on a given sensor; ISO mostly affects how the captured signal is amplified.