Like title says, I want to know if there are some official tutorial videos on Youtube or other platforms to learn Decker. It's easier for me to understand new things through videos rather than a manual.
Not currently! Though the idea gets talked about sometimes.
I don't know if this would help but there's also Phinxel's Field Notes which a bit more colorful and conversational. It covers some of the same information that's in the manual but it presents it in a different way.