How are transactions collected into new block? How are the latest transactions collected into new block to send them to miners for hashing? Which part of the network is responsible for giving miners the correct block to hash?
The mempool as it is called, stores transactions that are propagated by the peer to peer network.  Miners have the ability to select which transactions they include in the block.  It is beneficial for the miners to collect as many transactions as possible so they can obtain the most amount of fees that they can.