How a transaction is permanently accepted by the blockchain?

Question

If a miner A creates a block containing a transaction A and a miner B creates a block NOT containing the transaction A about at the same time, then some of the other miners will first receive the block of the miner A and the others will receive the block of the miner B. (Fork)

If the chain of the miner B grows faster than the chain of the miner A, then the transaction A will never be accepted (like it never happened). So, how the user A who created the transaction A can be sure that the transaction A was accepted/verified by the blockchain?

Also, is there a possibility that the transaction A is verified by the blockchain at some point and then in the future another subchain (from a previous fork) not containing the transaction A will grow faster, thus rejecting transaction A ?

Are there any time checkpoints created that all the miners should agree upon ?

Answer 1

If the chain of the miner B grows faster than the chain of the miner A, then the transaction A will never be accepted (like it never happened).

The transaction A is still unconfirmed in the mempool for any nodes following the best chain building on miner B's block. Any subsequent block is free to include it. The best chain has only one block at each height. Any competing blocks that don't end being part of the best chain are irrelevant for the network's state. So, if the block mined by miner A ends up being in the best chain, transaction A gets confirmed in A's block. If the block mined by miner B ends up being in the best chain, transaction A is free to get mined in the subsequent blocks following the block mined by B.

Confirmation of transaction is always probabilistic. Theoretically, an arbitrary long part of the chaintip could be reorganized to a better chain. It's just increasingly unlikely the longer the tip is. Many users consider transactions with six confirmations to be reliably confirmed. For smaller amounts, they may even trust fewer confirmations, larger amounts may cause the recipient to wait longer.