Why does it seem that Surat Fussilat (verse 9) refers to the creation of the heavens and the earth in six days, while other verses suggest eight days, even though modern science mentions billions of years?

There is no contradiction between what is stated in Surat Fussilat and other surahs regarding the creation of the heavens and the earth, for the Qur'an does not differ because it is from Allah. The phrase "six days" is not mentioned in Surat Fussilat, but rather in seven other places, and Surat Fussilat elaborates on what was summarized in those surahs. "Day" may refer to a period of time whose duration is known only to Allah. It could be six ages, or cosmic cycles, or phases that creatures went through, not our days that are linked to the solar cycle. These days are of the unseen, known only to Allah, and the purpose of mentioning them is to show the wisdom of divine decree and the cosmic order. As for "eight days," it is mentioned in Surat Al-Haqqah in the context of the destruction of the people of 'Ad, and it has no relation to the creation of the heavens and the earth.