Linux

For the #4, the file extension can be seen just as a note, a little tag that'll help you (or anyone else that will receive your file) remember which program you should use to successfully open the file.

From the viewpoint of your computer, in fact, a file is just a sequence of bits and every program can open every file, only it will not be able to find what it expects and actually do something useful with it, just as you can open a book written in any possible language: in most cases you will unable to undestand it, in some others you will be able to read it without any problem.

The "concept" of extensions was than introduced to allow your file manager (Explorer for Windows, Finder for macOS, Dolphin for KDE or Nautilus for GNOME) to know which program to launch when you double click on a certain file through a simple association table (that you can edit in your system preferences).

In regards to Linux you can sometimes read that file extensions are not a thing, but this is just because in the commandline you launch a specific program that you personally point to a certain file, so there is no file manager that needs to guess which app should be launched to open the document you just double clicked on.

That said, I think that should be pretty clear that in a Desktop context (like in a Personal Computer) that double click on a file situation pretty much applies to Linux too, so extensions will be useful and respected by the file manager you'll find installed in your distro of choice, even if it can use other means when that is missing.