To find the workshop items (maps/AI) you downloaded, navigate to your:

Steam > steamapps > workshop > content. There might be a few folders or only one, depending on the number of games for which you downloaded mods.

Go to this folder (221380) [it contains the Age of Empires 2: HD workshop files].

Find the map/AI you want to port, copy it, and paste it onto your desktop or somewhere easily accessible for later use.

To port the AI you created or downloaded, navigate to:

Steam/steamapps/common/AoE2DE/resources/_common/ai

Place your AI files there.

AI mods come with two files: one with something.AI and something.per. The .per file contains all the scripts, while the .AI file enables the AI to appear on the AI list in scenarios or when opening multiplayer games.

Take the .per and .AI files and put them into the "ai" folder mentioned above.

Porting Scenarios is a bit more complicated, since Age of Empires 2:HD and Age of Empires 2:DE has different file formats.

While Definitive Edition supports HD edition's file format.
HD Edition does not supports Definitive Edition's file format.

Navigate to:

C:\Users\YOURNAME\Games\Age of Empires 2 DE\76561198933361948\resources\_common\scenario

Place your scenarios in that scenario folder.

NOTE:[76561198933361948] These numbers might change, but it always this long it doesn't have to has the same numbers as I pasted here.

And no, copy-pasting your scenario to normal scenario folder does not work. It will simply not show up on the scenarios list in-game.

Thats it, now you can play any scenario or play with any AI you want on Definitive Edition.

If you have any problems or questions, feel free to ask me on comment section.