how are you? Quorum does not change how this validation is performed in regular Ethereum and consensus in this case is not relevant for the validation part.
To perform a transaction on an Ethereum chain, one must have a valid Ethereum account. An account involves a set of public and private keys which could be managed by the node itself or externally (through a wallet like MetaMask) and connectivity to the said node.
When generating a txn, if you are using a node managed account, you must request that node unlocks this account for you and then generate a txn with web3.eth.sendTransaction or the smart contract wrapped equivalent of. In the externally managed case, you would use something like sendRawTransaction in which case you must provide a signed payload to the node.
In either case, the txn will eventually end up in the node that accepted this txn and it will run additional state checks through the EVM. If they pass, txn is placed into txnpool and will be gossiped to other nodes in the blockchain until it reaches a node that is able to generate a block. Once it does, the block with this txn will be mined and distributed to all other nodes.
Quorum consensus mechanisms change who and how these blocks are generated (that is which nodes are allowed to generate block, how often a block is generated, etc) and do not add any additional checks on the initiators of txns as long as they are proper Ethereum accounts on the given Quorum chain.