I believe many time the boundary of a peninsula are politically defined, for instance most would often see the Iberia peninsula end at where Spain meet France, Indochina peninsula's boundary will probably be the southern border of China, and Sinai peninsula's boundary would be the current border between Israel and Egypt.
Other time there could be natural features that separate peninsula from the mainland it connected to, like a mountain range or a low lying corridor, or man-made structures like canal. However there are also situation that peninsula are separated from mainland using simply a straight line, depend on coast direction of surrounding land.
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