Say there are two minersA&B and each of them have produced the next block almost at the same instant. In this situation how is the network going to decide which one is the main block and abandon the other block?

Also, say if the txns contained by the first block are T1,T2,T3 while txns in the second block contains T2,T3,T4,what is going to happen to the uncommon transactions if either of the block get abandoned, i.e.T1or T4?

I am sure that from the txn pool, multiple miners would be picking up the same txns and start racing for the next block creation and insert that txn in the new block.