I didn’t see any composite video cables in that video. He’s using component video cables, specifically HD Retrovision’s Genesis and SNES cables, which take the native RGB output from those consoles and converts it to YPbPr.
The NES only outputs composite video at best, and the N64 only outputs S-Video at best; so HD Retrovision’s cables won’t work for you. You’d need to do one of the following:
A. Install RGB mods and get the correct cables;
B. Get a RetroTINK 2X, which you can either connect directly to your HDMI TV or pair with an HDMI->VGA converter to connect it to the OSSC; or
C. Get a Koryuu (whenever they get released) or another composite video/S-Video to YPbPr component converter, which you can connect to the component video input on the OSSC as shown in the video you linked.
You can, of course, just get a cheap converters on Amazon, eBay, or AliExpress–like a composite video/S-Video to SCART converter if you want to keep using the OSSC, or a composite video/S-Video to HDMI converter if you want to connect directly to your TV–but they won’t be without their problems, such as added input lag, black crush, poor color conversion, and/or questionable scaling methods.