What is the difference between creating a new poll and selecting an existing one?

When you add a Poll block, you are offered two buttons.

Create new poll starts a fresh poll with no votes. Use this when you are asking a new question.

Select existing places a poll you have already made, together with all the votes it has already collected. Use this when you want to bring back an earlier poll, or show the same poll in more than one place.

The block also tells you how many polls are currently in your library, so you can see at a glance whether you have anything to reuse.