The man page for ascii(7) helps.
While it doesn't list the combinations such as
^J, it lists
@ and the uppercase letters, then
_, on the right column, aligned with the control symbols on the left column.
The former (
@, uppercase letters and the other 5 symbols) have ASCII codes
0x5f, while the control characters have ASCII codes
0x1f. The Control key masks bit
0x40, so the end result of using it with those letters on the right is the corresponding control character on the left.
(The Control key also masks bit
0x20, which explains why lowercase characters, which are on the
0x7f range also produce the same control characters.
0x40 | 0x20 = 0x60 so that's what gets masked.)
^J is NL, newline, also represented as
"\n". It is the character used to indicate the end of a line in Unix file format.