Instant flows run in the name of the user who triggers the flow. If the flow is using premium features, each user triggering the flow will require a premium license. If a Power App triggers the flow the license can be a per app or per user license.
If within the flow, you want to identify the user who has triggered the flow, read my post Identifying the Flow User to find out how to do this.
When a user runs a Power App, by default, any records are created in the user’s name. Similarly, if a user runs a Power App that triggers an instant flow, all records created by that flow are in the name of the user. This ensures consistency in record ownership. However, if required, this can be changed so that records are created in the name of a service account or principal.
Always associate instant flows with the app that triggers them, particularly if they contain premium connectors. An associated flow runs using the premium Power App license of the person triggering them, and hence an associated flow won’t require the user to have an additional Power Automate license. Associating automated and scheduled flows to an app can also have licensing benefits. When associating flows with an app it is preferable for the flows and the app to be in the same solution to avoid dependencies.
Instant flows should also be shared with the users who will trigger it. If a Power App has been shared with users or a security group, then also share the flow with the same recipients.