block builders: professional players who are good at building high profit blocks, except for picking transactions from the memory pool of ethereum, they also receive private transactions from mev searchers. you can check the list of builders here: https://etherscan.io/accounts/label/mev-builder. Currently there are 16 builders. Builders build a block and submit it to the relays.
relay: relays accept blocks from builders, and pick the one with highest profit for the current proposer. You can check the list of relays here: https://github.com/remyroy/ethstaker/blob/main/MEV-relay-list.md#mev-relay-list-for-mainnet, there are 7 relays now. Theoretically relay can do evil things: it knows the content of blocks submitted by Builders, so it can re-build a block using that info and claim that this block is build by someone else; it provides blindBlock(only block header, no tx info) to proposers for signing, so it can fool the proposer by providing an invalid block and cause slashing to the proposer.
builder and relay are roles in the mev supply chain, some entity can play both roles at once. for example, Flashbots is both a relay and a builder.
reasons for separation:
- relays protect proposers from exposing, proposers only need to talk to a small number of relays they trust.
- the trust on relay and builder is different, the trust on relay is high, both proposer and builder need to trust a relay, while the trust on Builder is lower, it only needs to win the trust of mev searchers.
- division of labor facilitates professionalism
not sure about the context of Bankeless podcast, will happy to take a look if a link is provided