In natural covid the spike protein is on the outside of the virion, and facilitates the virus bonding with your cells and injecting the viral rna into your cells. Then your cell machinery gets hijacked to produce more covid until the cell bursts and releases the virions to infect more cells.

The mrna vaccine hijacks your cells to produce the spike protein only, not the full covid virus. The intent being to train your immune system to mount defense against the spike protein, which in turn causes it to mount defense against the full covid.