Hi Guys,

What would be a good starting point to understand and learn how transactions are done on behalf of users. Like the exchanges.
User has own bitcoin address in say bitstamp/kraken/blockchain.info
The user has their own 24 words seed.
What I want to learn is how the different providers initiate a transaction on behalf of the user from the users wallet. Surely they don't store users passphrase, that sounds way too unsecure.

Any articles, github project, would be much appreciated.

Sorry for the basic question.