So does Ethereum let me run an uber-like taxi app for fractions of a cent and not pay my employees? How does it do that?

I think you fundamentally don't understand what Ethereum is trying to do if you are thinking like this. I suggest you read the documentation and learn about what the infrastructure actually allows you to do.

Ethereum will allow you to deploy a smart contract that enables taxi drivers to provide that service to taxi customers. You do not have any employees and rather the taxi drivers will decide whether they wish to use your contract or not.

For making money: you can create a token which allows holders to have shareholder rights. You create a governance board to which the token shareholders vote on matters related to the business (expansion, token buy-back protocols, etc).

You're thinking with a centralized mindset. This decentralizes the service so it's driven by the community rather than a company. No one has employees.

So the service itself runs for pretty much free, and the consumer only pays overhead of staking and any programmatically added fees (which is going to be much, much less overhead than Uber carries).