Regarding adenovirus vaccine. The difference is your body cells don't make the spike proteins by using mRNA to recode-reprogram your cells.
The adenovirus infects the cells and like all viruses do, they force the cells to make proteins, so with Jansen vaccine, your cells then make the covid spike protein.
But their is no mRNA involved, it is as if you got a normal viral infection.
It also takes longer time to get an immunity, couple of weeks. So your immune system is primed differently than with mRNA, which mRNA seems to me more brute forced, versus a more natural process of cell infection by a virus.
The Johnson & Johnson adenovirus vaccine explained - Mayo Clinic
What's done to the virus is that actually a little genetic snippet is exchanged and placed into the adenovirus that is actually from the COVID virus. It's the section of genetic code that codes for the spike protein. The adenovirus is like a Trojan horse, except that what it's delivering is a good thing, instead of delivering something that you don't want in your body. You get the vaccine, the adenovirus goes into your cell, it's got this Trojan horse code on it that makes the spike protein. That spike protein then goes to the surface of your cell and then your immune system recognizes it and starts to make antibodies to it.
In the end, what you get is your body makes the spike protein and you develop the immune response to that spike protein, exactly like you do with the Pfizer vaccine and the Moderna vaccine