Have you used DDO before? The mask is most likely just one of the smart masks generated from the baked/provided textures. There are numerous tutorials out there for DDO to get your workflow up to speed and quickly kick out things like that. For Outpost, they created a smart material that was applied to all of the assets so they all have the same paint, metal, etc., but still have their own unique scratches and wear from their unique normals, AO, curvature, etc. Super useful and quick for this.
For the AO node, like pje said, you’re connecting a vec3 to a scalar input. Connect one of the RGB channels instead of the white dot. The AO input of that function has a (S) beside it, which means it only accepts a single scalar value or channel.