I am new to blockchain technology let alone hyperledger fabric. But as far as I understand all the nodes or peers of other organizations are involved while application / user requests for update transaction. And if all the nodes of network agrees to the proposal, leger gets updated.
And I coudnt find when the nodes of organisations agreed to update transactions?
Am I getting it all wrong?
I would recommend you read the Fabric Key Concepts topics, you could start with the Peers topic.
In a nutshell, you can define the endorsement policy for any smart contract which defines which organization's peers must execute the transaction and agree on the results before it gets validated and committed to a blockchain channel.
If you usually see the organization's and it's corresponding peer's will be joining a channel and do all the transactions (any operation like create,query etc ..). There is a ledger for a channel and all peer's have a copy of that ledger.
For your question
And I coudnt find when the nodes of organisations agreed to update transactions?
Let's take A as an peer from organization Org1 tries to update the value in the ledger. ( So this can be basically called as any client(application) tries to send an request to update something in ledger)
Now this request, someone has to validate. That's where endorsing peer (these are the peer's where chaincode (buisness logic) is installed) comes into picture. Now this transaction done by A will be validated by the policies that are defined in chaincode.
You can refer about endorsing peers and policies here
