Data Storage in Btc Blockchain

Can someone explain to me how data storage works on the BTC blockchain?

Witness data and OP_RETURN?

Can this allow illegal content on the blockchain?

I read about the recent discussion regarding the Bitcoin Core update that would facilitate this type of data on the chain, potentially being a veiled attack on BTC as a whole (after all, storing illegal content on a computer, even if it's a node, is a crime and puts everyone at risk).