Large text selection doesn't work In macOS, one can select text quickly and precisely using shortcuts. 

Pressing SHIFT and the arrow keys will select one character at a time (← →) or one line (↑, ↓) at a time. And pressing ALT and SHIFT in combination with the arrow keys will let you select each word at a time (← →) or from the insertion point to the very beginning or end of a document (↑, ↓).



However, I am having a hard time making it work on a larger document:



Why isn't the shortcut working in the second situation?
⇧ shift ⌥ alt ↓ selects to the end of the paragraph. This works in your first example because the document contains a single paragraph.

The correct combination for selecting to the end of the document is either ⇧ shift ⌘ cmd ↓ or ⇧ shift end, depending on that particular application’s shortcut for scrolling to the end of the document.

Correspondingly, selecting to the beginning of the document is ⇧ shift ⌘ cmd ↑ or ⇧ shift ↖︎ home.

And finally selecting to the start of the line is ⇧ shift ⌘ cmd ←, and to the end of the line ⇧ shift ⌘ cmd →. Using ⌥ alt instead of ⌘ cmd selects the previous/next word, respectively.