So I conclude:
The winning fork has its own version of pending transactions (mempool) which will still have the transactions not included in the overtaking chain. The nodes which followed the shorter version for a while will not have these, but will get to know about them when these pending transactions eventually will be included in a block.
The transactions from the orphaned block that weren't included to the latest block(s) of the longest will be added to their mempool again.
The book: Mastering bitcoin used the term "queued up again" that lightly indicates that the node will move them from the orphaned block back to mempool.