Hi Byong,
Yeah, there are a number of free samples included on the marketplace, and you can use any assets in them in your own game. The engine itself also includes a set of starter content you can auto-include into any new project you make (some meshes, cool materials, a few particle systems, etc…).
Here’s a partial list of samples that are available: 示例与教学 | 虚幻引擎文档
Cheers,
Michael Noland