I’m a bitcoin noob, but I’m really interested in understanding how it works under the hood. Once concept I don’t really understand is the longest chain wins

Let's suppose we have two chains: Chain A, with: block 1, block 2, block 3, block 4, block 5 Chain B, with: block 1, block 2, block 3, block 4, block 5. My transaction is located in block 1 of chain B, so I should see my transaction validated correctly and arrived at the desired address. What happens if chain A "wins" and becomes the authentic Bitcoin chain? My transaction will turn “unsent” and back to the mempool or to my address? Do I have to pay the mining fees again if I want to sent this transaction? And the miners of chain B will still keep their rewards even though then mined blocks that won’t be part of the authentic bitcoin blockchain? Hope this make sense, thanks everyone