Necrozma was going through light withdrawls and acts as a parasite in its fusion. The dex entries, and the Ultra Recon Squad too I think, state that Necrozma lost too much of its light all the time and constantly searches for sources of it; the merged forms also state that it's forcing them to produce all the light non-stop and that it's "Devouring" the light.Something that bothered me a little is that Necrozma can fuse with Solgaleo and Lunala.
Kyurem fusing with Reshiram or Zekrom, that kinda makes sense since they were once Pokemon together. But how does Necrozma absorbing Solgaleo or Lunala specifically make sense? Why can it turn ultra when holding a Z-Crystal while being fused and not without it? Nothing in terms of lore explains it. Eternatus is responsable directly to Dynamax so you would expect it to be able to have a unique form of it's own. Also why does Coloress have 2 devices to fuse Necrozma and not just one that does both? It's very confusing. I honestly don't mind it ripping off Kyurem if it was well reasoned. It feels like the game designers thought it was cool and put it in the game without any thought.
The reason it can turn Ultra only when attached is because the base form has too little light to work with. Where as once attached to solgaleo/lunala there's plenty of light that when combined with its specific z crystal it can (briefly) return to its full power.
Basically if there was another legend that could hypothetically produce the light it needs, it could probably parasite off that too (but wouldn't, because gamefreak probably doesn't want to bother making a bunch of forms any time they touch the concept again).