I looked up the J&J vaccine info and it is NOT an mRNA. It uses a different delivery system.
The J&J uses mRNA via DNA you might say. Moderna and Pfizer deliver mRNA directly to your cell’s ribosomes causing them to manufacture the spike protein. J&J adds a step, namely it uses an adenovirus to deliver DNA to the cell’s nucleus which then creates mRNA that causes the ribosomes to manufacture the spike protein. So what they have in common is they all “trick” your cells into producing the spike protein by causing the ribosomes to receive virus encoded mRNA. The difference is they do this either directly or via an extra step.