I used to be able to open a unity project by double clicking on a scene from the windows file explorer, but now doing so only opens the HUB and I have to add the project to the HUB before opening it. I’m a teacher and I have to open 30+ new projects every week. It’s waste of time to link a new project everytime and It’s even more annoying knowing it used to work fine. How can I fix this?
Well you tell Windows 10 which program to open when you double click on a file under “Start → Settings → Apps → Default apps → Choose default apps by file type”. It is probably set to the Unity Hub for opening scene files. You can try setting it to open an installed Unity Editor instead, and see if that works.
But Unity starting from 2019.1.0 requires the Hub, I believe to handle licensing. So I don’t know what happens if you try to launch it outside of the Hub anymore. Unity 2018.x and earlier don’t need the Hub installed at all.
Well it did work with 2019.4.18f1, but when I installed a more recent version of Unity, it would try to open with that editor instead. So I uninstalled the latest version to only have 2019.4.18f1 and I did set windows to open .unity file with that editor. But every time I try to open a file, it starts loading the Unity Editor (I can see it in the process), but then just opens the Unity Hub instead and stops the editor.
The program which opens when you double click on the file is a feature of Windows, not Unity. Did you read the first part I wrote?
this seems to be my default program setting for opening scene file, check what is yours:
personally i use my own tool to handle these,
https://github.com/unitycoder/UnityLauncherPro
it adds explorer context menu, so you can just right click inside folder, and launch correct unity version for this project:
Yes, i did specify to open with the editor and it does start to launch, but it seems to crash and open the Hub instead : 
Oh thanks for your tool, I’ll give it a try!