Searchers find the opportunities, validators execute the order

'Searchers' run specialized software constantly scanning the mempool for profitable MEV opportunities, then submit bundles of transactions (often via private relays, not the public mempool) designed to capture that value. Validators (or block builders working with them) choose which bundles to include, typically favoring whichever pays them the most in fees or direct payment.

This has formalized into infrastructure like Proposer-Builder Separation, where specialized 'builders' construct optimized blocks and validators simply pick the most profitable one offered to them, separating the job of building a block from the job of proposing it.