blockchain – How do miners know how to eliminate recently confirmed transactions?

A miner is in the process of creating and extracting a block of 1400 transactions. Suddenly, they receive a transmission that a new block was confirmed with 200 of the transactions currently in that miner's block. How does a miner proceed in this scenario: eliminate the 200 and add another 200 from the mempool and try to exploit a block?

In a second scenario, two blocks are created almost at the same time and each has some of them and some different transactions. I assume that (a) there is a race to consensus for each one and the first to 51% wins a confirmation. If so, then similar to the previous one (b) the block with lost transactions has all the dissimilar transactions recovered in the mempool. But what happens if a second chain emerges with two blocks? Now all the confirmations in (a) that are not confirmed in this second block are put back in the mempool and are expected to eventually be extracted in the block chain?

The way for me >>> Obviously this system works since it is very popular! But it seems that transactions can take a long time and there is no way to really guarantee that they will occur unless a payer decides to pay a high fee to almost confirm that the miners will pay attention first to their transactions and the majority will include that transaction in the following Miner's block to add to the chain.