Transfering Sims 4 packs between different electronics.

You are buying the product from a specific storefront, for the device that storefront supports. These devices tell you outright that this is how it works, and you agree to that on purchase. It's really that simple. 

you cannot transfer a console game to pc. only on xbox and some selected titles allow this. for example forza horizon series.
you can merge the ea app+steam+epic games pack. you need to install the game on  a single shared folder. for example:
A:\mygames\the sims 4 in that folder you install ea app/steam/epic games sims 4 and the DLC you have purchased. once you boot the game from any store all the DLC will be avaidable to you. you need to have the ea app account linked with steam and epic if you plan to merge all the stores purchases or else it wont work. aka all the sims 4 and dlc must be under the SAME ea app account.