I'm currently working on an anvil following Blender Guru's anvil tutorial. I'm at the unwrapping UVs part (https://youtu.be/scPSP_U858k?t=807), but for some reason when I select one part of my island, it instaselects another island that should not be "entangled" together. I think it's because of the seams, so I tried changing them a bit which made the unwrap look crazy. What method can I use to fix it? I'm guessing this will depend on my specific mesh so here's the blend file https://pasteall.org/media/c/9/c96420b7be91f3cd54f57af9ac4cca86.blend
You have select sync turned on. Nothing is wrong here. The mesh components are touching in 3D, and since that option is on, blender shows the whole UV map. Any pseudo-duplicate vertices that are on the border of two patches and thus part of both get selected as one.
Turn off select sync to avoid undesired effects from this.