Multi Strings by Freemusicproduction.net is a Virtual Instrument Audio Plugin and Soundware (samples or presets that load into other products) for macOS and Windows. It includes, and is therefore "powered by", HALion Sonic SE, which functions as a VST Plugin, an Audio Units Plugin, a VST 3 Plugin and an AAX Plugin.
The OS and Format icons below are for the latest version of HALion Sonic SE. The version numbers are for Multi Strings.
For those who didn't watch or notice it in the video, this requires HALion Sonic SE player...which is free but IMO sucks, so I'm passing. Too bad; I like what I see of the demo for some "synth strings" and similar sounds.