What part of Microsoft 365 security center do you use to identify users and devices affected by alert?

Use Incidents. 

The additional tabs for an incident are:

Alerts - All the alerts related to the incident and their information.

Devices - All the devices that have been identified to be part of or related to the incident.

Users - All the users that have been identified to be part of or related to the incident.

Mailboxes - All the mailboxes that have been identified to be part of or related to the incident.

Investigations - All the automated investigations triggered by alerts in the incident.

Evidence and Response - All the supported events and suspicious entities in the alerts of the incident.

Summary - A quick overview of the impacted assets associated with alerts.