MeshCombiner.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class MeshCombiner : MonoBehaviour
{
    [SerializeField] private List<MeshFilter> sourceMeshFilters;
    [SerializeField] private MeshFilter targetMeshFilter;

    [ContextMenu("Combine Meshes")]
    private void CombineMeshes()
    {
        var combine = new CombineInstance[sourceMeshFilters.Count];

        for (var i = 0; i < sourceMeshFilters.Count; i++)
        {
            combine[i].mesh = sourceMeshFilters[i].sharedMesh;
            combine[i].transform = sourceMeshFilters[i].transform.localToWorldMatrix;
        }

        // Create a new mesh and set its index format to UInt32
        var mesh = new Mesh();
        mesh.indexFormat = UnityEngine.Rendering.IndexFormat.UInt32;

        // Combine the meshes with the UInt32 index format
        mesh.CombineMeshes(combine);

        // Set the combined mesh to the target MeshFilter
        targetMeshFilter.mesh = mesh;
    }
}