I have this problem that i don't understand 3d strategy games mouse system. How do i get the tile where the mouse is on?

Some form of simple raytracing, perhaps some variation of the colition system used in lesson 30 would do it, but this will not be your largest problem.

You'd think somebody would write up some kind of universal link to this topic, I see like 5 of these a day on this forum.

3d pointing. A good writeup on all aspects of converting a 2d to 3d point or using matching methods to check the hit.

you'd think so.
but the fact is that there is no real magic mouse point/select solution.
For every situation, for every game you have to choose a method and then do some adaptation to it.

Some game require you to only select a tile on a flat grids, some need pixel perfect acuracy and some use something in between.

So it is best to define your game before you decide the details of how to conrol it.