If you look at it that way, you're almost right. The 64 GB are installed internally. The 128 GB would then be on a memory card that you have to buy separately and insert into the memory card slot of the phone. Note that you then have 2 drives like on a PC: 1x internal memory 1x SD card memory, but this is solved the same for every smartphone except for Apple (these do not have extra SD slots).

However, the operating system + pre-installed apps are deducted from the 64 GB of internal memory. How large this is varies from manufacturer to manufacturer, but 64 GB of internal memory is quite a lot.

If you don't install 200 apps, that's easily enough. You can then use the 128GB SD card to store photos, videos, etc. and larger data on it.