Why can I not make a subfolder in photo’s?

But I do not understand why it is in a closed package and why I can not access them directly?

Because of a couple of reasons:

1. Photos is a parametric editor - it never touches your original files, but rather stores the edits you make in its database. If you want to access your images for use in an email or another document, and you want the edited version, being able to access the files on the HD will not help, as all that will get you is the original. So, share, export etc. So, it only makes sense to do that for the originals too. (Incidentally, this is equally true of all the parametric editors, Lightroom, Capture One etc)
2. Up until the the point where Apple made that a closed package, about 90% of the responses on here were attempting to help people who went into the library via the Finder and "fixed" things to their own preferences, thereby corrupting the library. They renamed files and folders, the added and removed things and then wondered why Photos (or its predecessor iPhoto) didn't work anymore.
3. The files are renamed in Photos to help manage them in the cloud, so they all have unique names. Again the sealed package protects not only the library on your Mac but also the one in the Cloud and all the devices linked to it from user activity.

On the other hand, you can, if you prefer, and have a fondness for pain and pointless chores, run what's called a Referenced Library. In this set up the files are not copied to the library package, but rather left where you have them on import.

But...

In this scenario you can't use the Cloud Photo Library.

Also:

Photos has no tools to manage images stored outside the package. So this is where the pointless chores come in: Everything is more work for you: You move the images from the camera to your preferred storage. Then import them. Deletions? Delete the image in Photos and then go to your storage and root out the deleted image yourself. Anything like moving your store to another disk to a new computer becomes a world of pain because - and this is where the pain really comes in: If the path to the file is broken for any reason you will need to reconnect it manually. Not really a problem if it's one photo. But if it's 10,000 or 100,00, well that's a lot of pain indeed.

All those other managers - you know the ones you pay for - have tools to manage a referenced library. Relocating files and folders is simple, so is reconnect files and all that, because these apps have the tools for that, and that's one of the reasons you pay for them. If that's important to you, then Photos is not the app for you - it's not for me, for instance, I use a different more capable app.

My rule of thumb: If you want to use Photos don't use a referenced library, if you want referenced library don't use Photos.

Photos is storing the photos in a Photos Library, a closed package, that the users cannot access directly using the files.app. Even if we could, we would not be able to find and recognize the photos there, as Photos is renaming the files internally with unique names.

To access your photos, use the Photos.app. You can create folder inside Photos and create albums inside your folders to organize the photos. Edit, share, and organize albums in iPad – Apple Support (UK)

• Albums in Photos can hold media items, like photos and videos. Albums are flat without an internal structure.
• Folders can be nested and hold subfolders and albums, but to contain media items. To have photos in a folder we have to create an album in the folder.

We have to create albums directly inside the folder where we want them to appear, because albums cannot be moved between folders. So it is best to create first the nested folder structure, then create our albums inside the folders.

Why come back now, months after the query is answered, to post that irrelevancy?

This absence of simple organizational tools in the native app can be a drawback for users seeking efficient simple photo, and do not forget image, management.

A common mistake is to assume that Photos is meant to be all things to all people. It's not. It's a give-away app squarely aimed at the iPhone shooter who, frankly, doesn't want to do much photo management. Hence it automates as much as possible so the user doesn't have to think about those chores. Just because it will also do other things doesn't mean it'll be especially good at them, but it is good at what it is intended for - a family snapper taking pics and sharing them to friends and family. Photos is not the best photo management app out there. It's not even the best photo manager Apple ever made, and that is by design.

Apple giveaway a lot of software that is "good-enough-for-a-lot-of-things" but not world beating. Pages is handy if you want to lay out a local newsletter or drop a note to the local school. But if you're trying to write a doctoral thesis you're going to want to use Word, Nisus Writer or Mellel. iMovie is a handy video editor, but the options offered by people like Adobe are better (as in Apple's own Final Cut). And so Photos is a useful photo manager and parametric editor but no match for either version of Adobe's Lightroom, CaptureOne, On1 Photo Raw, DxO Photolab or Mylio, et al. But that's a healthy ecosystem. Here's a pretty useful app that does x,y,z... you want a,b,c too? Then purchase or lease a more powerful app from a 3rd party. So there's space in the ecosystem for free, inexpensive and expensive apps, depending on your requirements.

You can only judge an app by how it performs against the maker's intentions and its price point. On that basis Photos is good, and the price is right. But if that's not enough for you then it's time to seek out other options. The good news is that the healthy ecosystem means you ahve other options to explore.

I don't know how many people I've had to explain to (people who are otherwise some what knowledgeable about computers) that it is actually possible to export your photos from Apple's photos app so you don't have to keep paying Apple more and more and more for iCloud storage. This is why.

To actually answer it

Next time try read the question.

sick burn, bro 👍