This idiom or expression has to do with:

Making a decision.
Reaching a conclusion.

A. Why are you smiling?
B. I’ve come to the conclusion that we’re both idiots!

This idiom or expression has to do with:

Putting the facts together in order to understand the problem.

A. There were many indications that he was planning to run away. How did you miss them?
B. Sorry, we screwed up. Nobody connected the dots!

Also:
Drawing conclusions.
Seeing the big picture.