There's actually quite a few companies that make small turbojets especially for military purposes and up to a certain point, you can even DIY your own from spar

There's actually quite a few companies that make small turbojets especially for military purposes and up to a certain point, you can even DIY your own from spare parts (there are some Youtubers that have done that). That's the first tier and includes companies like Williams, usually producing <5,000 lbf engines. Then you have companies like Garrett/Honeywell that can make engines that output tens of thousands lbf like the TFE731 for mid-range jets and fighter jets. These can power big jets but not very fuel efficiently; that's the second tier. The next tier up is the high bypass turbofans producing tens of thousands lbf used for commercial aviation and afterburner engines for last gen fighter aircraft are a completely different story. The GP is right there are only a few players in the game. Due to scaling laws, at this point it becomes less about the design and more about the metallurgy and material science. Magic like single crystal alloys are critical here and are very closely guarded secrets because the knowledge unlocks everything from ICBMs to gaseous centrifuges to nuclear reactors.