Does a youtube channel with my lectures add to my portfolio in graduate applications if the response is good? Will teaching physics on a youtube channel and receiving a good response on it help me with my grad school applications in the subject?
I think this would depend on the location of your institution and how traditionally/modern inclined its culture is, as well as the person itself who reviews your application, but it certainly wouldnt hurt your application in any way. It is certainly a good way to stand out from the rest of the field, since I highly doubt any other applicant to your same programme would also be doing it.
If your graduate programme you are applying for includes teaching or TAing duties then it would be particularly relevant since it shows you already have good skills in explaining concepts to people who are less familiar with them than you are.