As I understand, all the voting machines in Georgia are of the BMD type which means the voter enters their vote through and electronic device which then prints out their vote. This “ballot” is then fed into a reader. I’m not sure if the vote tally is done from the entry device or if it is done on the actual printed ballot. But basically the machine could print one result on the ballot in text that agrees with the voter and then the machine can print out a barcode or QR code that is actually read by the reader that indicates a different count. Then when that ballot is put through any machine, it would count up the same every time and the voter would be none-the-wiser.
I’m not really sure how this is handled but I have hear that the BMD machines are very hackable.
You can read this analysis done by a Berkley student that talks about the BMD machines in great detail and how they can be unverifiable:
https://www.stat.berkeley.edu/~stark/Preprints/bmd-p19.pdf
Does the voter ever get to see the generated paper ballot to determine if it accurately reflects his choices?
If the choices are encoded in a non-readable form, there is no way to determine if this encoded string (bar code) is correct. But a manual recount would not read this encoding.
The only way I see the fraud occurring in light of possible recounts is if the generated paper artifact reflects the altered ballot.