DynamicCreateMesh.cs

using UnityEngine;

[RequireComponent (typeof(MeshRenderer))]
[RequireComponent (typeof(MeshFilter))]
public class DynamicCreateMesh : MonoBehaviour
{
    [SerializeField] Material _colorMaterial;
    
    private void Start ()
    {
        var mesh = new Mesh ();
        mesh.vertices = new Vector3[] {
            new Vector3 (0, 1f),
            new Vector3 (1f, -1f),
            new Vector3 (-1f, -1f),
        };
        mesh.triangles = new int[] {
            0, 1, 2 
        };
        mesh.RecalculateNormals ();
        var filter = GetComponent<MeshFilter> ();
        filter.sharedMesh = mesh;
        var meshRenderer = GetComponent<MeshRenderer>();
        // MeshRendererにMateriaｌをセット
        meshRenderer.material = _colorMaterial;
    }
}