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I have recently attempted to move a light switch on one of my walls by extending the cords a bit. At first I attempted to use one wire instead of 2 to save me some work

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and it would not work till I replicated the previous setup with actual 2 wires (simplified pic shown below). Can someone explain to me why?

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Another weird thing here is that the switch took 2 hot wires, which is unusual to me, but I guess it is something I need to lookup instead of asking a double question here.

P.S. I realize one (unrelated to the question) mistake that my initial decision introduced: I decreased the power capacity of that particular place by 50%. Feel free to criticize and whatever. I just want to understand that and to learn.

Edit: I don't know the wiring scheme of the house, but if I would swap the hot wires from 2 top cables, it would work, but it would also control 2 other switches on a neighboring wall (i.e. the switch would have to be on for those lights to function ;)

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    It's not clear from your diagrams what you changed. The two scenarios are functionally identical. Also, you can't "diminish capacity" with wiring. You either have a circuit or you don't, and the circuit breaker or fuse limits current. I'm not sure what you mean by that.
    – isherwood
    Commented May 7, 2018 at 19:38
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    The switch doesn't take two hot wires. It merely lets one pass through it (or not, if the switch is off). It seems like you have some fundamental misunderstandings of wiring and electricity and you may be putting yourself and others at risk by doing this work.
    – isherwood
    Commented May 7, 2018 at 19:42
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    The switch takes one hot wire and one switched-hot wire. I mark my switched-hot wires with colored tape (red, orange, brown, whatever will distinguish them from other wire functions in the work area), but that's not code or even necessary, just me doing me. If you can tell em apart, that's fine. Commented May 7, 2018 at 19:49
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    @isherwood is right, these two configurations would behave exactly the same way. Which means if one worked and the other didn't, then you didn't actually connect things the way you drew them. Photos would probably be more helpful than drawings at this point. I have a feeling there is more to this than what you've shown in your diagram.
    – CactusCake
    Commented May 7, 2018 at 19:56
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    Your edit tells us that the line voltage coming in is on the top left cable. When you reversed the top two cables, you were making all three loads require the switch to be ON to function (passing through the wire nut after the switch and splitting three ways). In the current configuration, the line voltage goes to the wire nut first, then on to two always-on circuits (the lower two cables) and one switched circuit (upper right)
    – CactusCake
    Commented May 7, 2018 at 21:51

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