This question often bothers ManyChat newbies :) Here's the thing: the Flow is a set of Send Message/Action blocks connected with one another. Once the flow is created, it only exists in the Flows section and it's not yet attached to any Growth Tool/broadcast/sequence etc. So this flow has no way of getting to your subsribers. However if you attach this flow to a Sequence, for example, and subscribe people to this sequence, the Flow in question will be delivered to them as a part of this sequence.
Thus Sequence may contain many different Flows. You can also attach flows to your Growth Tools, Keywords, send them as Broadcasts to your subscribers. So Flows are basically content units that you use in various tools in your ManyChat account.
We hope this helps clarify the matter! :)