TLDR; - Concrete is poured into a form, for structural members. Cement is spread as a glue.
Cement is a glue. Concrete is a mix of aggregates, held together with cement.
If you're building something structural (to support a load -- such as itself), you'd buy the concrete. If you're using it as a skim/finish coat, or to attach firebrick, buy the cement, and (possibly) mix it with some very fine aggregates - depending on the use.
See this answer for more detail on cement/concrete in general. What actually happens if I vary proportions of cement and sand in mortar?
EDIT: After seeing the video, I now understand. What you are making is a light, weak concrete using the cement and perlite as an aggregate. You need to buy the fire cement, not the concrete. (You could use concrete and omit the perlite, but I think it would be pretty heavy.)