The entire TV uses Roku for the user interface. Roku is not separate. You either use Roku apps for your sources (Netflix, Hulu, Amazon Prime Video, YouTube, Disney Plus, etc.) or you can get content through a HDMI input source (TiVo, Blu-Ray, Fire TV Stick, AV receiver, or though OTA antenna or composite video (think VCR). You can use streaming apps through other hardware such as BluRay player or Fire TV stick, but I don't see any real advantage to that, and I certainly wouldn't call that "directly".
Think of it this way. You use Android or iOS for your phone. Your computer uses Windows, MacOS, or possibly ChromeOS. This TV uses Roku. I find that it works very well.
My TV is hooked up to TiVo, Blu-Ray, Comcast X1 streaming box (so I can get Peacock, which will be available directly on Roku July 15, 2020 so I can ditch to X1), my AV receiver, and finally my VCR.
Roku has hundreds of additional apps that can be downloaded for additional content.