This is how PhysicalFileSystem works, exactly:

https://github.com/umbraco/Umbraco-CMS/blob/75c2b07ad3a093b5b65b6ebd45697687c062f62a/src/Umbraco.Core/IO/PhysicalFileSystem.cs

If you want to implement your own, you'll need a class that inherits from IFileSystem and then you implement all the methods.

using System;
using System.Collections.Generic;
using System.IO;
using Umbraco.Core.IO;

namespace MyNamespace 
{
    internal class MyFileSystem : IFileSystem
    {
        public IEnumerable<string> GetDirectories(string path) { // implement me }

        public void DeleteDirectory(string path) { // implement me }

        public void DeleteDirectory(string path, bool recursive) { // implement me }

        public bool DirectoryExists(string path) { // implement me }

        public void AddFile(string path, Stream stream) { // implement me }

        public void AddFile(string path, Stream stream, bool overrideIfExists) { // implement me }

        public IEnumerable<string> GetFiles(string path) { // implement me }

        public IEnumerable<string> GetFiles(string path, string filter) { // implement me }

        public Stream OpenFile(string path) { // implement me }

        public void DeleteFile(string path) { // implement me }

        public bool FileExists(string path) { // implement me }

        public string GetRelativePath(string fullPathOrUrl) { // implement me }

        public string GetFullPath(string path) { // implement me }

        public string GetUrl(string path) { // implement me }

        public DateTimeOffset GetLastModified(string path) { // implement me }

        public DateTimeOffset GetCreated(string path) { // implement me }
    }
}