The J&J vaccine is actually very similar to the Pfizer and Moderna. What they have in common is that cells produce the virus spike protein through mRNA, and then the police cells develop antibodies against the spike protein. If the actual covid virus enters your body, the antibodies that you created will block the spike protein and prevent the virus from entering the cells. The difference is that Pfizer/Moderna inject the mRNA directly, while the J&J vaccine injects a virus that has a gene inside that is the recipe for making the mRNA. So the virus goes inside a cell, dumps the gene inside the nucleus, the nucleus produces the mRNA, and the mRNA is used to make the spike protein. The advantage of the J&J is that the adenovirus is much more stable than the mRNA, therefore it doesnt need the super low temperature.