SARS CoV2 coronavirus is spread primarily by aerosol transmission. SARS (the original coronavirus in 2004) was determined to have aerosol transmission when a patient in a Hong Kong hospital was infected by a SARS patient in a different room ACROSS THE HALL.
Your dirty fingers have little to do with spreading covid unless you rub your eyes, which have very little biological defense compared to to your throat and upper respiratory system.
And your eyes are not protected by your mask. But the exterior of the mask quickly becomes laden with virus every time you exhale.