Mev-boost, a piece of software that consensus clients delegate their block building to, is currently being used on mainnet. The difference is that there can be multiple block builder _relays_, such as Flashbots, bloXroute, Manifold Finance, and Blocknative.

These relays effectively accept bundles from MEV searchers, which are then sent to the validator running mev-boost as they propose their block.

https://boost.flashbots.net/