I can answer a bit about the other two!
x-wave originates mostly from chillwave, which was a genre of music back in the 2000s that was basically taking stuff like electropop, new wave, shoegaze, etc, but with a more relaxed, nostalgic vibe. The actual term itself was coined as a kind of joke about how a lot of this music sounds like the stuff you'd hear from 80s tv marathons or vhs tapes or what have you. it was one of the first musical genres to develop primarily online (compared to a lot of the stuff that inspired it), and was pretty much the origin point for a lot of other stuff that would follow, most notably vaporwave, which is literally just chillwave but, like, ironic.
x-core also has musical origins, specifically hardcore punk music, which was distinct from regular punk rock because it was a lot faster, more aggressive, and "harder"- leaning closer to stuff like metal. When this genre kept expanding a lot of musical artists kept the 'core' part, mostly because of what had influenced them, which is how we started getting stuff like metalcore, synthcore, breakcore, and so on- its kind of like how there's all those genres of music with 'step' at the end (techstep, darkstep, etc), because they share a similar origin point but use the first half of the genre to differentiate themselves.
The thing is, these differences were pretty much only important to actual musicians who wanted to make it really distinct what they were trying to go for, whereas the average listener isn't really going to care what the differences between drum and base, jungle, breakbeat, or happycore are.