I'm not sure how much this will help, but this is a quote from Wicke in Sun/Moon regarding Nihilego:
"Its most distinctive feature is its parasitic capability. When
Nihilego latches on to a host, it does not manipulate its actions
directly. Rather it awakens the host's own capabilities and boosts
them to an extreme extent in order to protect itself. It injects the
host with a sort of neurotoxin to achieve this effect. This neurotoxin
of Nihilego's is incredibly stimulating and inspires feelings of
extreme excitement and a lack of inhibition in its host. In other
words, anything or anyone that a Nihilego latches on to will have its
native skills forcibly activated to their fullest extent and will then
act as it naturally desires to."
So according to this, Nihilego uses a neurotoxin to affect it's hosts behavior. We also learn though Lusamine's condition at the end of the game that the lingering effects of this neurotoxin leaves the host very ill. How it actually physically fuses with them is something that I suppose is up to the imagination. However in the official character sheets for the UBs it says that Nihilego is "a flexible glass jellyfish", so I'm guessing that it is easy for Nihilego to "leech on to organisms" (as Stakatacool said in the comments) given it's physical characteristics. In said character sheet we also see Nihilego latching onto Pikachu sort of the same way it did with Lusamine, it seems to me that what it does is grabs it's prey and pushes them up into it's bell (head).
If you want to take a look at the character sheet for yourself you can see it here
