Please do tell us what you find.
I agree that the white one with the plug is your soil pipe cleanout.
How far from the house is this? Is it close enough for a gutter downspout? You say you heard an echo from the open one when a car ran over a manhole cover. Does the manhole cover say "sewer" on it or possibly "storm" or "storm drain" ? If it does say storm, considering that the pipe is above the ground, it's probably for a gutter.
The one with a broken lid looks like a shutoff to me. Pop the lid off. Do you know where your shutoff is? You should find it in case you get a leak on the wrong side of a valve and need to shut your house off.
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