You have to hold the Shift key while moving a window for it to snap into the tiles you set up. If you just move them normally they have a different snapping behaviour like what you described.

Edit: So as the deleted reply was probably asking, this is how it works in full. If you have the KDE Plasma desktop environment after a certain version (I wanna say 5.6-ish?) you can do the following:

1. Press Windows+T, or as we Linux nerds like to call it Meta+T, to configure your "tiling zones" on your monitors.
2. Hold Shift+LeftClick on the title bar of a window to move them into the "tiling zones" you set up.

Discoverability on this sucks (as much of the Plasma desktop does) but it's a pretty cool feature.

How do you get into that tiling config? I found it at one time but now I can't seem to figure out how to bring it up.

Edit: found it, Meta-T.

And then shift dragging works for snapping.

@danjay have your cursor snap like at the middle - i think it is like 'bro is real low so he wants to snap down in this corner, right'