How can one determine if all reagents will react in the reaction? Is it correct that all the reagents above the arrow will participate to form an intermediate product, to which the reagents below the arrow are then added, and they react to form the final product?
Thank you in advance.
That is correct, the two numbered reagent and solvent pairs refer to two reactions, that will be done after one another.
There is an intermediate product that can be isolated, or at least identified.