I moved on to Steam from another platform, where is my additional content?

Additional content & marketplace purchases are not cross-platforms.

When you purchase additional content from the Marketplace or when you redeem an Upgrade (Deluxe or Premium) the entitlements are automatically redeemed onto your gamertag for that platform only.

If you own Microsoft Flight Simulator on both the Microsoft Store and Steam, you will only be able to access the additional content on the platform used to make the purchase, even if you use the same gamertag on both platforms.