I bought RW6 - do I have to pay to watch the training videos (stacks?)
Of course. The videos are offered as an optional additional training/resource to learn more about RapidWeaver.
There are hundreds of training videos out there for free - try a search on YouTube or this on Vimeo https://vimeo.com/search?q=rapidweaver
Many add-on developers offer instructional videos on their product pages. I found NimbleHost’s videos to be especially educational.