Updated Aug 11
Selection tends to be a numerical issue. It is largely about who survives their environment long enough to reproduce efficiently. Which leads to large numbers of progeny and descendants.
Without getting into a debate as to whether they are “really alive”, the most numerous species on the planet are bacteriophages. They are viruses which infect bacteria. They are very small, and relatively simple (although they are actually more complex and elegant than they seem at first).
The next most numerous are bacteria and archaea, which are comprised of single cells. These domains contain a range of species that can survive a very wide range of environments. Each individual species has its own needs, but, as general groups, bacteria and archaea can be found in many places that more complex life forms cannot.
Simplicity vs complexity relates to the amount of time and energy that is required for replicating yourself. A human being takes nine months, while bacteria have a theoretical minimum of 20 minutes (although, in reality, it tends to be somewhat longer, like maybe an hour, depending on conditions). The required energy and nutrients are vastly less per new bacterium.
Some species can evolve towards being more simple. If you are a microbe in an environment where certain needed molecules are plentiful, then you can lose the genes that code for enzymes to make those molecules yourself. Then, replicating yourself takes less time and resources, so you are now more efficient.
Microbes were around for a very long time before humans, and will continue long after we are gone.
Some multicellular organisms are also advantaged by relative simplicity. There are far, far more insects than there are mammals, for example. Also, their progeny are independent immediately, rather than needing years of parenting, sexual development, etc.