Struct passed into a function carries changes outside function scope

I have a method:

public void AddQuad(Vector3Int blockPosition, Quad quad)
    Quad inQuad = quad;
    for (int i = 0; i < 4; i++) inQuad.Points += blockPosition;

Where Quad is a struct containing a Vector3[] called points
and Quads is a Quad[] in the same class as this method
when this method runs, it behaves as though the quad parameter was given as a reference.

_inQuad.points += blockPosition;

actually changes the values in the quad that was passed in, instead of only adding a modified version to the array Quads. Why is this happening? I thought passing a struct as a parameter created a copy of the struct, and didn’t alter the original.

Arrays are reference types. A struct which contains an array just contains a reference to the array.