For this thread consider the smallest unit of life to be a breeding population.
As long as some some of the breeding population survive and it produces offspring, then life has succeeded.
Every breeding population has some advantageous mutations, as evidenced by those that survive and breed better than others.
Every breeding population has some disadvantageous mutations, as evidenced by those that don't survive and reproduce or do so marginally with little input to the gene pool.
Every breeding population has some neutral mutations, as evidenced by apparent lack of mutation that either help nor hinder the population from breeding.
Every breeding population is subject to stochastic events that have nothing to do with fitness, but result in random chance altering the makeup of the breeding pool available for reproduction. (Genetic Drift)
A basic definition for evolution that I've developed is: Evolution is the side effects of life doing what life does. No special processes, no special forces. Just life carrying on and going about its business.
How life started has no effect on the matter. Evolution is what life does after having gotten started. Same thing if all life were supernaturally created and same thing if life developed through purely natural processes with absolutely no supernatural influences and same thing if life developed through purely natural processes set up and influenced by the supernatural. If doesn't matter how life got started; as soon as that happened life started evolving.
So to understand how evolution works, first understand how life works.
The theory of evolution (ToE) is that this is sufficient to explain all the diversity of life as we know it.
A sideways addendum if I may:
In the beginning the reproduction that takes place is a chemical imperative without any choice.
Got matter? Got energy gradient?
Electrons detach, attach in accordance with the electro-chemical state present in the surrounding stuff. Collections of atoms change their positions and detach/attach with other collections of atoms.
Then some configuration is achieved that fosters the gradients/matter to (more or less) produce copies of the original configuration.
As long as the supply of stuff and energy gradient lasts, perhaps millions upon millions of the self-reproducing configuration are produced, some with a minor difference, some with a major difference to the original. Configurations more/less productive, more/less efficient, reproduce at differential rates. Any matter/energy environment so suited continues the process.
So, survive … reproduce … repeat. Complex chemistry in continuous motion.
Life has happened.
Everything that has followed (pre-proto-cell walls, swallowing mitochondria, multi-cellularity, jaws, shells, phalanges, diamond rings, jobs, Häagen-Dazs Chocolate Ice Cream and postage stamps) are all the result of this original chemistry creating novel configurations of stuff that now seem to have taken on a strong zeitgeist to continue to reproduce to this day.
No purpose. No goal. Just the mindless propensity of chemicals to obey the one true god of the Universe – Physics.
And, before anyone asks, yes, Häagen-Dazs Chocolate Ice Cream can have a positive effect on reproductive success.