My understanding is that all miners are feeded with transactions published by full node servers, but I didn't understand why they end up with exactly the same transactions in their created blocks ? Why miner A doesn't produce a block with saying 4 transactions and miner B produce a block with, for instance again, 3 transactions ?
Nodes will relay all transactions to each other, true. But, how do you know all miners have the same block if only one is actually published? In any case, miners are incentivized monetarily to include the transactions with the highest fee rate (fee/kB) because the miner gets all of the transaction fees. So, it would not be surprising if miners had similar blocks since they all would presumable try to maximize the transaction fees.