Does it count as stealing if I use fanfics for a..fanfic?

I know it might sound confusing but I want to explain it more... I've been obsessed with those "X anime react to their multiverse" and I'm quite considering on writing one but instead of being them as other anime characters,they react to different fanfics like they're another reality. I hope I explained it well,but I really want to know ToT. Does it count as stealing? Is it allowed?