Mutations occur by chance. With trillions of bacteria, mutations are a constant. Each possibly mutation is happening somewhere. Natural selection then dictates

Mutations occur by chance. With trillions of bacteria, mutations are a constant. Each possibly mutation is happening somewhere. Natural selection then dictates which mutations thrive when the environment favors them to do so. Evolution is a system, not a lottery. There is also a meta layer to evolution, that organisms can "evolve to evolve", that the rate of "random" mutations can itself be an evolved trait. An organism unable to evolve, say with locked-in genetic code, will not adapt. Sexual reproduction arguably is such a system: an evolved system that regulates future evolution. So even base mutations may not be totally random.