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Make vast level landscapes, puzzles, UI elements, effects and physics environments. Export that all to an open JSON format. And then use any engine or programming language that fits best for your game in order to bring it all together.
There is currently no export or import option in overlap2d, will not be in the future as the developer wants to make it independent of any engine.
So, it's the later case, provide an importer for your engine. Like spriter or spine. They don't export engine specific data, instead engine developers provide runtime/importer for their own engine.