You clearly seem to have absolutely no understanding about, what the word "Necro" means, do you ?
In case, that you have really no clues..
Necro is
latin and means death, phage is
instead a greek word and means eating. So basically, Necrophages are something, that doesn't exist, as its just a compounding of two words from different languages - however, if you put simply both words together, you can think of Drowners of being undeads (because thats the species of what you call something, that is in fact DEAD, and yet still able to move around and do things, as if its still living!!), which eat the flesh of the living which they kill that are so uncautious to go into the territories, where the drowners lurk around in the swamps and lakes/seas where people died from drowning, to turn into drowners through a virus, because thats the other biology meaning of the word phage, as in like
Bacteriophage
https://en.wiktionary.org/wiki/phage?rdfrom=Phage
Wraithss
are GHOSTS, in Witcher Terms not undeads, thats why you use against them also the oil, that lets you deal more damage agaisnt ghosts, same as like when you fight a Noonwraith or Nightwraith.
Sure, by common logic you can see ghosts also as "undeads", but you would have to differentiate here then between corporeal undeads with a body and incorporeal undeads, that consist just out of Ectoplasma and a strong Soul, that binds the emotions of a dead person to the wordly existance as long the soul hasn't done something specificly important, while the person' was still alive to be able to rest in peace.
Anyone who played the Quest "A Towerful of Mice" should understand this...
An undead with a body instead is either a summon of a Necromancer or a result of a powerful curse, who/which puts souls into dead corpses so that they can use/turn them as/into vessels to get bonded to the worldly existance as long as the Necromancers holds them there or the curse doesn't get lifted by a Witcher in this case.
Or they are result of a parasitic virus/fungus, that turns people into walking dead., what would be the biological non mangical way to become an undead.
Drowners are no result of magic, Drowners are a result of a virus that invades the body of a dead drowned person, what makes them Necrophages in a biological way. in Witcher terms spoken. However in genral fantasy terms spoken, they are nothign else but simply undeads, cause it are dead creatures, that can still walk and do things, despite being dead as they should be supposed to not be able to move at all, what is the very definition of "undead"
https://en.wikipedia.org/wiki/Undead
However, enough with the arguing over undeads now, as thats not realyl the topic of this thread.