The sequencer pool ensures that the members are legitimate and meets the requirements for entering the pool (e.g. stake amount). Before any transaction is sent, the current sequencer needs to be assigned to the specific range of blocks that they can submit. >2/3 of the sequencers in this pool need to agree before the batch is accepted and sent.
The validity of transactions are determined by the sequencers signing via MPC before pushing to L1, this verifies the transactions that are included in the batch.
The order of the transactions are determined by the Block Proposer. This is rotated based on the Consensus (POS) layer. This process reduces the centralization risk for MEV.