This is not a support forum for games made with unity.
Its normal for bigger mobile games to download needed content after installation. The iOS and Android app-store have size limitations for the initial installation size, so downloading the big game asset from a external server is the way to go.
The extra download could contain most of the game’s asset(like textures audio and meshes).
No you can’t technical and especially not legally use the files that come with there game in your game.
Need a starting point for getting into unity and making games?