I'll give you the example of the land border crossings between Hong Kong and China.

At Lok Ma Chau/Huanggang, the busiest crossing, there are flyovers (or overpasses, as we call it here in the US).

At the older Man Kam To crossing, the bridge across Shenzhen River runs like roads in Hong Kong (driving on the left). Once you cross into China, you turn left into the Chinese border control area to the west, and then exit to other roads. Coming back, the Chinese border control area is to the east. So, you just drive into that area, and when you approach the bridge, you'll be driving on the left side. No major flyovers there.

Same thing at Sha Tau Kok.

At the new Western Crossing opening today, the long bridge across Shenzhen Bay again runs like Hong Kong roads - driving on the left. I believe the roadways cross over itself where the bridge hits Chinese soil.

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BTW, I really like the pretzel-style roadway at the Chinese end of the Lotus Bridge between China and Macau. You can see it very clearly on Google Earth.