Having knowledge in power electronics i can confidently say the DC output of solar is easily and regularly inverted in phase with grid. In fact, DC is often used for undersea cables switching AC to dc then back to AC, All at extremely high voltage and varying demand(up yo 600kV/600MW but varying by installation).

Wind turbines go online after the blades start spinning and connect to the grid in the same way as any other generator, controlled by internal electronics. Power is regulated through blade feathering and can be turned off as supply exceeds demand. This, other than for maintenance reasons, is why you might see one turbine spinning while the next is standing still. This capability actually means the grid is MORE stable with wind power.

Any further fluctuation is managed in the same way as conventional power generation.