Okay, so SARS-CoV-2 carries these viral fusion proteins (think of them like cellular lock jimmies) on the spike protein (remember that shit?), and it uses this to bypass the membrane and enter the cell. Once inside the virus doesn’t need its jimmies anymore, so they’re discarded in the cell’s membrane. This is bad because the jimmies still work, so now these lock picks are lodged in the membrane and can latch on to other neighboring cell membranes. When this happens they fusion dance into syncytium, which are basically giant cells with multiple nucleus.
While syncytia (the plural of syncyium) are actually normal in tissues like muscle cells, when that happens in your brain or lungs its bad. At best this just kills the cells, but if they don’t die they just kinda hang around and can’t do their jobs and cause microclots.