Defender for endpoints & Azure Arc for servers

[Pa_D]

Question:
To monitor onprem servers (2012, 2016, 2019) can we just install MMA agent or do we need to onboard server to Azure Arc & Azure security center?

[#2]

You need to onboard Servers to Azure Arc & Azure Security Center so that the security logs are collected in ASC for monitoring.
Hope this link helps
https://docs.microsoft.com/en-us/azure/azure-arc/servers/overview