StreamingAssets/Resources Extensions for Unity

ResourcesExtension.cs

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

public static class ResourcesExtension
{
    /// <summary>
    /// Recursively traverses each folder under <paramref name="path"/> and returns the list of file paths. 
    /// It will only work in Editor mode.
    /// </summary>
    /// <param name="path">Relative to Application.streamingAssetsPath.</param>
    /// <param name="paths">List of file path strings.</param>
    /// <returns>List of file path strings.</returns>
    public static List<string> GetPathsRecursively(string path, ref List<string> paths)
    {
        var fullPath = Path.Combine(Application.dataPath, "Resources", path);
        DirectoryInfo dirInfo = new DirectoryInfo(fullPath);
        foreach (var file in dirInfo.GetFiles())
        {
            if (!file.Name.Contains(".meta"))
            {
                paths.Add(Path.Combine(path, Path.GetFileNameWithoutExtension(file.Name))); // Without file extension
            }
        }

        foreach (var dir in dirInfo.GetDirectories())
        {
            GetPathsRecursively(Path.Combine(path, dir.Name), ref paths);
        }

        return paths;
    }

    public static List<string> GetPathsRecursively(string path)
    {
        List<string> paths = new List<string>();
        return GetPathsRecursively(path, ref paths);
    }

    /// <summary>
    /// Recursively traverses each folder under <paramref name="basePath"/> and loads each file as a <typeparamref name="T"/>. 
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="basePath"></param>
    /// <returns>Returns the loaded object of type <typeparamref name="T"/></returns>
    public static List<T> LoadRecursively<T>(string basePath) where T : Object
    {
        List<string> paths = GetPathsRecursively(basePath);

        List<T> objects = new List<T>();
        foreach (var path in paths)
        {
            objects.Add(Resources.Load<T>(path));
        }
        return objects;
    }

    public static List<(T, string)> LoadRecursivelyWithPaths<T>(string basePath) where T : Object
    {
        List<string> paths = new List<string>();
        GetPathsRecursively(basePath, ref paths);

        List<(T, string)> list = new List<(T, string)>();
        foreach (var path in paths)
        {
            list.Add((Resources.Load<T>(path), path));
        }
        return list;
    }
}

StreamingAssetsExtension.cs

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

public static class StreamingAssetsExtension
{
    /// <summary>
    /// Recursively traverses each folder under <paramref name="path"/> and returns the list of file paths. 
    /// It will only work in Editor mode.
    /// </summary>
    /// <param name="path">Relative to Application.streamingAssetsPath.</param>
    /// <param name="paths">List of file path strings.</param>
    /// <returns>List of file path strings.</returns>
    public static List<string> GetPathsRecursively(string path, ref List<string> paths)
    {
        var fullPath = Path.Combine(Application.streamingAssetsPath, path);
        DirectoryInfo dirInfo = new DirectoryInfo(fullPath);
        foreach (var file in dirInfo.GetFiles())
        {
            if (!file.Name.Contains(".meta"))
            {
                paths.Add(Path.Combine(path, file.Name)); // With file extension
            }
        }

        foreach (var dir in dirInfo.GetDirectories())
        {
            GetPathsRecursively(Path.Combine(path, dir.Name), ref paths);
        }

        return paths;
    }

    public static List<string> GetPathsRecursively(string path)
    {
        List<string> paths = new List<string>();
        return GetPathsRecursively(path, ref paths);
    }
}

Example usage: https://stackoverflow.com/a/73303644/4735573