External RAM isn't a thing.

No, you can’t add RAM over USB. That’s not how computer memory works. USB is way too slow for system RAM, and even if it weren’t, the Chromebook’s hardware simply doesn’t allow it.

Cloud storage is also not RAM — it’s long-term storage (like a hard drive in the sky). You’re mixing up two different things. RAM is what your computer uses to actively run programs right now. Cloud storage is where files live when you aren’t using them.

Your N4120 Chromebook with 8GB soldered is what it is. There’s no upgrade path. If you’re running out of memory, close more tabs, or consider a Chromebook with 16GB next time. You can enable zram (usually on by default) and maybe use a fast SD card for swap, but that’s a band-aid, not a fix.