geo/Mirror

The plane is given as a point on it and a direction across it, both in world space, so the defaults reflect across X through the origin. An input that has been PLACED, by a geo/Transform in OBJECT mode, a geo/Copy or a parent, is reflected where it stands rather than where its points happen to be stored. Points move to the other side, normals turn with them, and the face winding is reversed, which is what keeps the reflected copy solid rather than inside-out.

With keepInput the node emits BOTH halves, which is the usual way to build a symmetric shape: model one side, mirror, done. Without it the input is replaced by its reflection, which is what the node has always done.

The two halves are separate objects and their points are not welded along the plane. Put a geo/Fuse after it when the seam has to close, for a print export or a boolean.

An SDF field is reflected too, by the same plane. There the halves are two objects carrying two fields: put a geo/Merge in compact mode after it to fold them into one hard union. geo/SDFMirror is the other operation on a field, and it is not the same one: it FOLDS the field across the origin planes, keeping the positive side alone and costing no duplicated subtree.