She is 78 and thinks she is more tech savvy than the average person because she can check email, and use twitter and Facebook, and does a regular blog on Wordpress. She has been firmly entrenched (she thinks) in Windows since forever, but in reality she has very little understanding of how or why anything works the way it does, and has a very hard time adapting to change and remembering new ways of doing things.
The macOS User Guide would be a good start although it’s just documentation rather than videos.
ThisIsE has a good playlist for beginners as does MacMost although both are a few years old.
You can also pick up Guy Hart-Davis’ “macOS Sequoia For Dummies” which is a great book for beginners.