Most of the people doing speed level designs have a collection of assets already made, that’s what makes them so speedy. Megascans is like $20/month I think for the low version, but if that’s too much, JonF nailed it. Epic has provided a ton of content, for free, from their previous projects. If you want something realistic, look for the Paragon and Kite Demo assets.
Also, if you want to play around with particular games’ assets, look at the Modding tab in the Launcher as well. You can download a few different ‘modded’ versions of the engine, usually outdated compared to the standard version, that has all the respective content for the game. I think you’re more limited in that aspect to only use those assets in that version of the engine.