You have an Entra ID tenant, you just don't know it. Entra is a prerequisite to the Defender XDR portal. You can use entra.microsoft.com or the Azure portal to explore. You should find your E5 licenses in the portal under Entra > licensing.
MDE does not perform license validation. Your end-user devices are part of the E3 or E5 license. These is a separate license for Windows and Linux servers. You can purchase a stand-alone license for servers for a reseller or used Defender for Servers (which includes MDE).
Entra is also required for Office 365 and Intune.