Large text selection doesn't work
⇧ 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.