Developers are building applications on top of Bittensor subnets and APIs. Developers might be a subnet owner, validator or a miner, but it is not a requirement. There is no $TAO requirement to be a developer (Sone subnets may require TAO as payment).

A developer is likely to just register an API key, and use the subnet output in their service.

Examples of developers building on top of Bittensor are: 18 and 19.