I will try to reply to the best of my knowledge.
Windows Defender on Windows versions lower than 8 it's NOT A COMPLETE AV program. That's because Windows Defender has become somewhat of a misnomer. On w7, for instance, the Windows Defender included was just an antimalware program. Nothing else and nothing more.
On Windows 8+ Windows Defender is a full-fledged (or almost full-fledged) AV package that can compete with most Free AVs out there and even with some paid ones. It does its job. It has a small performance footprint and it's really free (it won't nag or pester you with any popup messages saying you need to pay to get all the bells and whistles).
And I think Windows Defender (which has evolved a lot over the years), it has got a behaviour blocker (AKA "block at first sight"). I think it's enabled by default if leaving WD configuration options at default (real-time protection, cloud lookup and such).
Edited by eLPuSHeR, 13 July 2018 - 04:33 AM.