Do magnetic fields play a role in the revelation of the Message and the composition of the Qur’an, and does fearing this doubt constitute shirk (associating partners with God) or kufr (disbelief)?

We advise you to remember Allah, seek refuge from Satan, and recite Ayat al-Kursi and the Mu'awwidhatayn. Magnetic lines cannot compose the Quran and reveal it. Your fear of disbelief indicates that you are not a disbeliever; rather, these are whisperings from which no believer is safe. The Prophet, peace and blessings be upon him, said: "Praise be to Allah who reduced his cunning to whispering."