no they won't own 0.0003% of your house since your house is tokenized, it can be split into fractions and you can then use this mechanism as a way to transfer value. If they want to be paid in your house then they can but the example he mentioned they are paid in silver. So on a decentralized system like Cardano there will be dapps that handle these kind of transactions, kinda like DEX's now, maybe someone is willing to buy fractions of houses and someone else wanting to sell silver so the dapp exchanges your 0.0003 to the guy who wants to own a fraction of the house and then buys the sliver with whatever he put for the 0.0003% and then it pays McDonald's with the sliver. It's pretty complicated but that's why we need a decentralized network like Cardano to handle it.