We did multi platform apps when we still called them programs. We used some cross platform toolkits with either cross platform UIs that felt out of place everywhere or a layer that hooked to a common subset of the native ones, with maybe an option to go really native. We wrote C or C++, mostly.

Then HTML became that common subset of the UI and the browser JS runtime and rendering engine replaced the cross platform toolkit. Bloated, not optimized for space and time as it could be, etc, but very convenient.