This is where you are wrong. You have probably heard of base ISO on a camera. Thats is the actual physical sensitivity of the sensor or say the optimal and ONLY working ISO. Say your camera's native ISO is 400. Whenever you press your shutter button the sensor captures the same information at its native ISO 400, disregarding your ISO setting on the camera. The change to your choosen ISO on your camera is applied by the processor, which will then digitally lower exposure values for ever pixel if you shoot at the setting ISO 100 (400->100 = 2 stops of lower exposure) or it will digitally increase the exposure value if you choose to shoot at say ISO 1600.
You can basically disregard your iso setting on your camera these days. Of course I oversimplified things here. Because the internal processor uses optimized algorithms developped by the manufacturer of that sensor, which of course will always give you better results in the noise handling areas when shooting at higher exposure settings compared to when you yourself adjust the exposure levels in post, say in lightroom. Programmes like these, wether for pics or video can do a really good job, but they are not optimized for any particular camera model or their sensors. But if you're not pixelpeeping you won't see any difference.
Then of course there is the matter of the newer cameras having dual ISO. Which basically means that you have 2 native (or base) ISO levels with your sensor. Often it is around 400ISO and the second is often around 1200, but those vary wildly from model to model. This of course gives you way more flexibilty. You got 2 real exposure settings and therefore digital exposure intervention is reduced greatly.
All that being said, I put iso-less in " ", because very few cameras are actually and really iso-less. Most of them come somewhat close others less, but still... you are not ever changing your physical ISO of the camera unless it is a dual iso capable camera, it's always digital adjustments inside or outside of the camera. This is why you see so many people with technical backgrounds arguing that the ISO setting on your camera is a misleading name nowadays and should rather be called "gain", as in digital signal gain. Of course, we're talking about RAW for pictures and video. If you shoot pics in jpeg or video in normal stand profile picture or a log profile all of this iso-less talk doesn't matter as the selected digital exposure and color temperature is backed in to the file.