Hi there @VHTme,
Thanks for posting in the Community,
To create a segment-triggered flow, create a new flow and select Segment from the Trigger Setup pane in the lefthand sidebar. It is something you can add to the beginning of a flow.

Next, you will be prompted to select the segment that will trigger the flow. Once you set a particular segment to trigger the flow, you will not be able to change it to a different segment. In order to do this, you will need to clone the flow and change the segment that triggers the flow.
More info can be found here.
All the best,
Alex