[1/13] It may be initially confusing to fully grasp how deposits and withdrawals from L1 to @optimismPBC are actually implemented, and it helps to see the on-chain action of what is happening behind the scenes.

[2/13] Initial setup (simplified): on L1 we have SyntheticBridgeToOptimism from Synthetic and OVM_L1CrossDomainManager from Optimism contracts. On L2 we have SynthetixBridgeToBase and OVM_L2_CrossDomainManager contracts.

[3/13] Additionally we have Sequencer (L2 mining node) that verifies all L2 transactions and submits them in batches to L1 for future reference and Relayer that is responsible for relaying messages from L2 —> L1

[4/13] Step 1 - Alice wants to deposit $SNX to L2. To do that she calls initiateDeposit() method on the L1 SyntheticBridgeToOptimism which takes her $SNX, puts it in the escrow and calls OVM_L1CrossDomainManager sendMessage() method.

[5/13] The CrossDomainManager puts this request to CanonicalTransactionChain (this is an “official” and “unmutable” list of all L2 transactions on L1). As a consequence the Sequencer (L2 mining node) will need to execute this transaction on L2.