True there is nothing to push against in space. But “pushing against air’ is how JET engines work. Rocket engines in space work by using the equal and opposite reaction to the expelling of matter (mass from the rocket exhaust) from the rocket engine. Just like if you were in space, and had a sack of rocks. By throwiing them down past your feet, one after another, you would propel your body up, faster with each rock thrown.
re: “But “pushing against air’ is how JET engines work.”
I think AgentJayZ on YT would disagree with you; he’s the jet engine guy who goes into the theory and practicality of jet engines.
Besides, ALL you’ve got to work with at sea level is ~ 14.7 pounds per sq inch ...
“Exhaust Pressure: Jet Engine Mistakes 2”
https://www.youtube.com/watch?v=E3nKpHWcJA4