Hi @Sang,
Glad I could help.
Your abandoned cart flow is triggered by a metric, Checkout Started. Because this flow is event-based, it only has one option. It back-populates based on the Placed Order trigger, specifically when the profiles in the flow made the Placed Order event. All the current emails in Needs Review won't send automatically when you set it to Live, you will have to go in and Manually send them (there's a Send All feature). Due to this metric triggered flows can only be back-populated as far back as the time delay. Example: If your time delay is only 24 hours, then only the profiles from the last day will receive the initial message. The rest of the profiles will wait and then exit the flow. To include profiles that started a checkout before yesterday, you will need to click on the time delays and change 'hours' to 'days' and set the desired number of days:

You can read more about it here: How Back-Populating Works in Klaviyo.
Hope this helped,
Alex