Ctrl clears the highest two bits. E.g. @ (ascii 0x40) turns into ^@/NUL (ascii 0x00). Similarly C (0x43) -> ^C/ETX (0x03).

Older terminals (like the VT05) used bit-pairing also with shift. They just flipped some bits depending on the upper two bits. Compare column 2 with 3 in the linked graphic, and 4,5 with 6,7.