Here is a [thrust equation](https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/thrsteq.html) from the NASA website, which seems to have perked up a bit since the success of the Artemis mission.

We see thrust is produced by *net* momentum change and net pressure differential from a given exhaust area.

With turbojets, higher pressure is created by compressing ambient air.  Heating it (by the relationship of Pressure and Volume being proportional to Temperature) allows both the exhaust area to be larger than the inlet and the exhaust velocity to be greater, producing net thrust.  There is only a slight increase in mass flow in the engine core (but adding fans or props greatly improves this in subsonic flight regimes).

The issue is that drag is created by the inlet and temperature increase is caused by compression, robbing the engine of its net thrust until a thrust/drag breakeven point is reached.  This is why some people claiming "hypersonic" aircraft these days are simply flying horizontal rockets, which are unburdened of the task of compressing adequate amounts oxidizer/mass from the atmosphere.  But, then again, that is exactly how the [X-15](https://en.m.wikipedia.org/wiki/North_American_X-15) managed hypersonic speeds back in the 1960s.

The limiting factor for jet propulsion is inlet temperature.  Use of cryogenic fuel and/or oxidizer to cool inlet air is one way of improving overall efficiency.