I'm creating a service for online fitness training. Users get course videos after payment, so I need to be able to close the access to the videos. So I have 2 questions:
If you want to stream the videos to multiple people on different devices connected over networks of varying quality, the usual approach is to use Adaptive Bit rate Streaming (https://stackoverflow.com/a/42365034/334402).
ABR basically creates multiple bit rate versions of your content and chunks them, so the client can choose the next chunk from the best bit rate for the device and current network conditions.
You may find it easiest to use a hosting service or, if hosting the video yourself, to use a commercial server (like Wowza, United, AWS Media Services etc) but there are examples of how to build simple ABR servers if you search also (e.g. at the time of writing: https://www.instructables.com/id/Making-Your-Own-Simple-DASH-MPEG-Server-Windows-10/)
For restricting access and protecting the content from being copied, you need to balance the effort/cost vs how important it is to make it hard to copy.
You can build simple anti-copying hurdles like authentication to access the page with the video URL, obfuscated URLs etc but ultimately if the video must play on the device it must be downloaded and there will be a way to copy it for a determined hacker.
The next level is usually to encrypted the content and to share the keys securely using some private channel or a recognised DRM system.
Remember that people can point a camera at the screen and copy also, but the quality is not usually good and it is time consuming. If this is an issue, you can't stop the camera recording but you can trace the user if you have invisible forensic watermarking and you can access the copied video.
