It's always a mix of electricity that comes out of the socket. However, you always get the electricity that is produced in your area.
Imagine being at a pool party, but the pool still needs to be filled. All guests help and fill the pool. Some people pour - for whatever reason - polluted water into the pool, others pour clean water. When you take water from the pool, you have a mixture of polluted and clean water. The more clean water you pour in, the cleaner the water in the whole pool will be. And the best thing would be if the water is completely clean at some point, so that the bathing fun can start!
It's the same with the energy that comes out of your socket. So when you buy renewable energy, you're helping to make the mix of electricity coming out of all your outlets cleaner than it was before. Yeah! And the more people get renewable energy, the more renewable energy will come out of your socket, until eventually our energy will be 100% renewable. So switch now and make the entire German energy mix cleaner with your decision!