You can't really make a general statement that will always be true. It will depend on everything from the actual CPU that has the integrated GPU, the external GPU, the motherboard, the BIOS, the OS, etc, etc. I know some combination of integrated GPU + certain external GPU + plus certain motherboards can actually be as you described and have the integrated GPU used for certain tasks and the external GPU take over from, or work together with the internal GPU. For other combinations, if the external card is plugged in the internal one is disabled. For others, you can pick which one you want to use in the BIOS, but you can only use one or the other, not both and it cannot be changed except through the BIOS. You'll just have to read the specs in detail, mainly the motherboard specs, and go from there.
Cheers and Regards