It’s the same as any other evolutionary process. Those who survive and reproduce the most carry their genes to the next generation; those who are less well adapted don’t reproduce as much and over the generations their genes get removed.
Say you have a bush with berries. Some bushes make sweeter berries than others. Let’s say those with the sweetest berries will attract more birds and animals who will eat the berries and scatter the seeds further away. The sweeter bushes are getting more lotto tickets for the next generation, the sour ones not so much. Repeat over time and eventually the sour ones will be gone.
The same can be applied to any other trait. It depends on the pressure of the environment they live in.