Connect the fuse box in the construction container

Electricity makes you small, black, and ugly. In this case, you are not far off.

Moreover, the components are already quite outdated.

To be honest, when I look at the mess, all my hair stands on end.

I wouldn't even know where to start listing the points I don't like.

Please have it done by someone who has learned this because, as you asked, neither you nor your buddy really know what you're doing; 12 V and 230 V are an immense difference.

Not much happens at 12 V. However, at 230 V, you could end up as a crispy roast.

By the way, a multimeter is not the right measuring device for this; you should use something that is really suitable for it.

Multimeter works if you know what you're doing. Otherwise: long live the [Duspol].