The J&J vaccine uses a replication-deficient adenovirus (virus can enter host cells/nuclei to deliver the payload but cannot hijack cellular machinery to make lots of copies of itself) as the delivery system.
The payload is double-stranded DNA that encodes the COVID spike protein. That payload is delivered to the host cell nucleus (the DNA does not integrate into the host cell genome) where it is transcribed into mRNA then translated to protein in the cytoplasm and ultimately presented to the immune system as foreign so a specific immune response can be at the ready if that antigen (the spike protein) is seen again.
The mRNA vaccines (Pfizer or Moderna) use a mix of lipids (like a cell membrane or micelle) to surround a payload of mRNA that encodes the same spike protein. This delivery system is capable of entering host cells and going into the cytoplasm where the mRNA is translated into spike protein to elicit the same immune response described above.
The inherent instability of mRNA means that it is degraded very soon after it is translated into protein in a cell.
The DNA of the J&J vaccine will last a little longer but will ultimately be degraded as well.
Neither of these are capable of changing your genome (DNA).
I skipped quite a bit of stuff, especially with the immune system, but that kind of gives an overview of the components and MOA of each.
If you want to know more about the components of each you can look at the FDA EUA documents linked below.
J&J (Go to section 13)
https://www.janssenlabels.com/emerg...n/Janssen+COVID-19+Vaccine-HCP-fact-sheet.pdf
Pfizer (Section 13 again)
https://www.fda.gov/media/144413/download
Moderna (again)
https://www.modernatx.com/covid19vaccine-eua/eua-fact-sheet-providers.pdf
Schmak, you probably already know all of this. I am just hoping to shed some context for others in case they were wondering what was in each of these vaccines.