2 PCs accidentally connected?

I recently set up a NAS in our home. I'm not a professional when it comes to this, but I've done it a few times but this time my parents' laptop and my PC are somehow connected. When I change my background picture, my parents suddenly have the same picture. How exactly did that happen and how can I undo it?

I just want to assume that you have linked the same Microsoft account on both PCs. I have that for example. On my PC and on my laptop - the same scenario.

Oops… That's actually true…