Does anyone know what these are stuffed with?

poly fill, like for stuffed animals, or you could use bits of poly batting

Do you think that the puffs could be made with strips or another was instead of individual puffs?

Not any I have seen.

I saw a video using strips. I think it might have been Kaye Woods, but I may be wrong.
http://www.cutoutandkeep.net/project..._biscuit_quilt
This site sort of explains it.
From memory, she marked on the base the squares for the biscuits. Sewed the long edge of the top piece with pleats to the base using sew & flip. Next sewed along each side of individual biscuits with pleats. Then she stuffed them and pinned long side down with pleats. Added next strip and sewed down.