First, let’s see a video showing in simple graphics how the ear works. It is a simple summary of how this amazing structure functions.
There are other videos in youtube where they go into more detail on how the cochlea works. This video is just a very general overview of the entire outer-middle-inner ear structure.
Now, I started with the video, because that was one of the first youtube videos I watched when I started my PhD. Beginning to read about audiology, I was a bit overwhelmed with all the terms and new information I had to take in. So I took to the internet to find answers that I could not really extract from scientific books as their descriptions (and my mind) were complicating things.
So this is exactly how I suggest to use youtube, or the internet in general, for your PhD. It can be a good way of finding simple and quick answers to help you understand notions, before you throw yourself back to the books and journal articles again. By no means you should rely on the internet to study your field. The material out there are not peer-reviewed and you should definitely not use it as a reference. So just find the right balance between internet use and scientific reading. Being critical while reading or watching stuff on the internet is always a good idea.
Another example I have is youtube videos that explain statistics. While I find them useful, I only use them as a reference and I use my judgment and always compare them to other reliable sources (statistics books, statistics courses). Especially for statistics, you can find different ways of doing analyses, and also different ways of interpreting the output. So you can use youtube to quickly understand how to do an analysis, but then you should cross-check with your course notes, or book notes and always follow a step-by-step process of analysing your data based on these latter two sources.