Question Creating a program that auto opens on computer sign in, runs in background, and opens pop ups.

I am making a calendar app for Windows, Mac, and Linux, and I wanted to add a secondary app that reads custom binary files that store events, then when it reaches the time that one of the events have in it, it makes a popup that has two buttons, one is a accept button, and the other is a mix of a snooze button and a input field that the user inputs how long to snooze for.

My main problem is this: I don't know how to make it auto open on computer sign in on Windows, Mac, and Linux. I can make the rest, however, when I looked into this problem, I came across this thread, where they mention a solution, but it seems overly complicated for what I'm trying to achieve, only works for Windows, and it seems that it doesn't work for graphical apps, like what I'm trying to make.

So I need a way to do it on Windows (If the thing I linked is a solution and I'm just not reading it right please let me know.), Mac, and Linux.

 

Can someone from Unity help me? I know that this is a very technical question.

 

Does it have to be made with Unity? Because Visual Studio, Electron and others seem better suited for what you're trying to do.

 

I've never made a UWP, and I'm not sure where to start. I've never heard of Electron, how hard is it to make what I'm trying to make in that?