Answers numbered as per your questions:
(1) I'm not sure why Angie is using the Ctrl key. If you've got an 'alternative graphic' (such as €) on a key, the usual way of typing it (unsurprisingly) is to press the relevant key together with 'Alt Gr'. That's exactly what that key's for!
(2) The 'Alt Gr' is used again.
(3) You've probably purchased an International QWERTY keyboard (or a variant of it), rather than a UK keyboard, and/or you've not got Windows set up to use UK English. You've not told us which version of Windows your computer is running. For Windows 7 or earlier, see here:
https://support.microsoft.com/en-gb/help/258824/how-to-change-your-keyboard-layout
For Windows 10, see here:
https://www.windowscentral.com/how-change-your-keyboard-layout-windows-10
(4) The key at the top left of your keyboard had a use in the days when you needed to write your own programs to get a computer to do anything. Theses days (apart possibly for use by a few coders) it's redundant. ' ¬ ', for example, is a 'logical negation' (or 'not', for short) symbol, which not many people need to use!