How can I credit to sources I'll use for my paid or free video tutorials on YouTube or Udemy?
The most general rule is that once you cite, you are no longer plagiarizing.  You can submit someone else's entire paper, and as long as you (very clearly) note that you didn't write a word of it, and that it was someone else's paper that you found interesting, you are no longer plagiarizing, as you are no longer representing someone else's work as your own.
The standards for citation are different in different settings.  In fully formal papers, you need to cite fact-by-fact, because every contribution needs to be traced directly.  In a video, it is sufficient to say that what you're presenting isn't developed by you, but really the work of other incredible people, and that you will link to those sources at the bottom of the video.  Again, the standard is whether you represent the work as your own or not.
A setting like an instructional video does not need (and typically does not support) each fact being separately cited.  Further, no one would expect it, any more than they would expect it in a lecture at university.
Good luck creating your course!