If you tell OBS to record the screen, which is the case if you use the "display capture" source, OBS will record whatever is on the screen. If Youtube is on the screen, OBS will record Youtube. If OBS is on the screen, OBS will record OBS. In its preview, OBS will show what it is capturing. If OBS is capturing OBS, OBS will show OBS - itself - in the preview. This is what you see with your "infinite number of screens".
To avoid this with display capture, you need to bring OBS into the background and the thing you want to capture in the foreground. Usually, people using OBS with display capture have 2 monitors and move OBS to the other monitor they aren't currently capturing, so it is out of the way. If you don't have 2 monitors, you can minimize OBS to avoid it capturing itself and start/stop recording via the OBS icon in the system try (lower right corner of your task bar). Additionally, you can define hotkeys with Settings->Hotkeys to start/stop recording and press a hotkey to control the minimized OBS.
Or you can remove the display capture source and explicitly capture an application. You do this by using a "window capture" source instead of a display capture source. In this case, only this application becomes visible in OBS. You might try this as well. Keep in mind that if you try to capture a browser like Chrome or Firefox, you need to disable hardware acceleration in the browser setup (not in OBS, in the browser setup) to enable OBS to capture what the browsers show.
Having said this, OBS isn't made to capture a browser to capture a Youtube video. There are better ways to get a Youtube video - as far as I know, there is a huge number of "Youtube downloader" tools available on the web.
