So the Mrna finds its way to a local cell encapsulated in liposome(fat). Once inside the cell the mrna(messenger ribonucleic acid) instructs the cell to produce a genetically modified protein which resembles the spike protien on the coronavirus. It has to be modified as a typical spike protein has the function of fusing the virus to the cell and that is not a desired trait of the vaccine spike protien because that would cause cells to fuse and could cause things like clotting(a known side effect). So long story short. The virus has multiple proteins but they only targeted the s.protein which is how the virus reaches out and grabs the cell. So the virus comes into a vaccinated person and meets resistance of antibodies against its s.protein. So the virus thru the tried an true theory of evolution is going to mutate its spike protein(delta variant) so it can bypass the antibodies generated against it's old spike protein.

I could go on but its alot of typing in my phone.