Iirc borg have transwarp as well as backup warp capability. And as to “how it works”: I was under the impression that, regardless of the type of warp reactor (federation dilithium/antimatter, Romulan microsingularity, whatever the borg use, etc) the main important output is a shitload of energy, and the thing generating the energy is largely irrelevant - all that matters is that an Alcubierre effect is generated, and serves as superluminal motive force.