I'm assuming you are talking about art. Art is the heart and identity of most games. It is best to make your own, or have a person or team to create graphics under your direction.
Smart use of pre-built assets can possibly lead to great results (PUBG), but usually will look like junk, even if the original art was high quality (PUBG arguably also looks like junk). It would be pretty rare to find assets from a single artist that would cover all your needs, so you would have to mix and match, and then the lack of cohesive art direction becomes pretty obvious.
If you don't have the resources to make original art, then obviously you'll have to make do with whatever you can get.