When you link platforms to an Epic Games Account, you have access to all the items on those platforms. For example, when you link your Steam platform to your Epic Games Account you will have access to all items on both accounts. You will also have the progress level of your Primary account. For more info, check out: How Do I Link My Epic Games Account to a Platform?
The progress level from your Primary Platform is the one shared between all linked platforms. Your Challenges progression is also from your Primary Platform. To change your Primary Platform, check out: How Do I Change My Primary Platform?
Merging Levels or Challenges from different platforms into a single account is not possible. For example, if you have 2 Steam accounts, it is not possible to merge the items into a single Steam account.
You can also trade items between different platforms. To learn more about player-to-player trading, check out: How Do I Trade With Other Players.