TLDR: boats
Suspension bridges at a basic level only required a rope to be pulled across a river, and that can easily be done with a boat.
Larger footings could be poured from a boat as well.
While there’s a lot of prep work and drilling down on a river bed for bridges today, it more ancient times they would likely have just floated a barge to the spot and dumped large quantities of gravel and large rocks to create a solid footing
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