Vim implementation of the full selection function

have been looking to see if there is a quick way to Select all , but the internet is a lot of repetition, and wrong, such as:
1, $y, what it is. To write will also be written: 1, $y, but I tell you, written: 1, $y is all copied, not all selected
DG, this is even more misleading, which is to delete the contents of the line to the last line where the cursor is located (including the contents of the line where the cursor is)
It's all over here. Select the correct answer:
Ggvg
Explain the above command a little bit.
GG Let the cursor move to the first line, in vim only valid, vi invalid
V is into visual (visual) mode
The G-cursor moves to the last line
The contents of the selection can be followed by other actions, such as:
D Delete the selected content
Y Copy the contents of the selection to number No. 0 registers
"+y copies the contents of the selection to the + register, which is the system's clipboard, for other programs