Hello @Chrystal,
Great questions!
Since flows are automated emails that get sent out in response to a specific trigger occurring, there would not be a way to manually put someone into a flow or cause them to be manually triggered for a flow. Unless a customer meets the flow conditions and triggers, they would not be eligible for the flow.
From your two flow examples, customers would only trigger Flow 1: Failed Orders when they triggered the failed payment trigger. Similarly, customers would only enter Flow 2: Customer winback flow when they trigger a placed order event. The available triggers you can use to build and initiate flows are based on Lists, Segments, Metrics, and Date Properties. At this time there is no trigger available to initiate a customer into a flow when they complete or reach a certain step in another flow.
Furthermore, when a customer triggers a flow, they would trigger the flow from the very beginning and would not be placed in the middle of a flow to continue onwards. I would recommend taking a look at the following articles to further understand how flow triggers function and how contacts going through each flow:
In addition, customer winback flows are a way to entice customers who were previously engaged to return and interact with your brand again or make another purchase. Because of this, should a customer triggered the failed order flow, receive an email indicating that their payment failed, then proceed to make their purchase again, they would already be placed into the customer winback flow automatically due to recording a Placed Order event. If the customer however does not complete their order after receiving notice of their failed order, then the customer would not be queued for the customer winback flow as there would not be an engagement from this customer. For further reading, you can take a look at the Creating a Winback Flow article and Objective Video: Turn On a Winback Flow video resources Klaviyo offers on this subject.
I hope this helps!
David