In order for the doctor to know how to do the medical procedure the person who became the doctor had to spend numerous years studying numerous medical facts in order to later become a doctor, in order to later diagnose the medical problem, in order to later determine the solution to the medical problem, and in order to later propose a particular medical procedure and to perform the medical procedure. Figuratively speaking, the person who became the doctor had to collect and study multiple puzzle pieces and to piece them together. My analogy of jigsaw puzzles is thus valid.
For example, in order to show the fossil evidence for evolutionary transition of humans from non-human animals, one needs more than one fossil. One needs multiple fossils representing multiple stages of the transition (multiple transitional features). Likewise to show the evidence of evolution from comparative anatomy a person needs to compare more than one common feature between two different species. One needs to compare many common features (as well as different features and somewhat different features) between numerous species, to show that various current species have a common ancestor and to show evidence for biological evolution.
There is also the evidence from genetics of existing species, the evidence from biogeography, the evidence from experiments in mutations of genes, the evidence of atavisms, the evidence of radiometric dating of fossils, and more. There is also the principle of consilience. When these lines of evidence are all considered together, while using the principle of consilience, biological evolution is shown to be true well beyond a reasonable doubt (much like jurors considering multiple pieces of evidence before making a final decision and casting a verdict) - and non-evolutionary special creationism is shown to be extremely unlikely to be true.
Regarding atavisms see https://en.wikipedia.org/wiki/Atavism .
Sea Breeze, it sounds like you are the one who needs to do some more research and/or to do much more critical thinking (from a scientific perspective) on the topic.