This question and its answer are somewhat technical. Components of biological systems are often complex proteins which together function to create structure (like muscle tissue) or to catalyze reactions (like enzymes). Many different proteins work together in multiple systems to create functions (like the blood clotting mechanism and the “motor” of the bacterial flagellum).
It has been discovered that at least fifty different proteins work together to create the structure and function of the bacterial flagellum. All of them must be present for the flagellum to work. To imagine that fifty different proteins evolved piecemeal over millions of years and finally came together simultaneously by chance to create the “motor” is preposterous.
Where do these proteins come from? We know that proteins are manufactured in the cell by coded messages embedded in another molecule called DNA. The coded messages consist of chemical letters that spell out words, sentences, and genes for the manufacturing of proteins that catalyze thousands of reactions in the embryological process to create roses, giraffes, and human babies. Complex information like that embedded in DNA is only known to arise as a result of intelligence, not chance (like software).
The argument of irreducible complexity in defense of intelligent design is irrefutable when these many layers of structure and function in living systems are considered.