Endpoint.microsoft.com is the MEM portal aka Intune. It is an MDM and MAM solution to manage end user devices. You can enable Defender for endpoint in Intune to connect into Defender Portal security.microsoft.com (new link) to sync endpoint data.
Within Endpoint.microsoft.com, there is an option called endpoint security. You will find AV, ASR, EDR, Disk encryption related device security profiles here that can be pushed down as policies to Windows 10 & later devices that are managed by Intune.
I hope this clears some of your doubts.