All a vaccine needs to do is show the immune system a (hopefully useful) antigen. Your immune system doesn’t know or care really how that antigen got there, just that it wants to kill it right now. That’s what’s being referred to as “receptors of the virus’ own spike protein”. That’s receptors on cells that mediate the immune system response and decide if it needs to murder it
For COVID, the spike protein is the antigen of choice. Pfizer and Moderna use mRNA suspended lipid so your cells will take it in and manufacture that spike protein to trigger your immune system. The Johnson & Johnson vaccine uses a modified adenovirus that’s been rendered incapable of replication and just makes your cells produce those spike proteins. In this it’s a protein subunit vaccine, and they seem to have modified yeast to produce the spike protein which they tend extract to produce the vaccine.
iirc this is the same kind of vaccine (or very similar_ as is used for Hep B. So it may have the same downside as that, where you need 3-4 doses for long term immunity.