Should we do sip investment through hdfc amc account or hdfc securities

I have recently open my DMAT account on HDFC Securities. I can purchase or shell share from here.
Even I can purchase SIP from here but it is regular growth not director growth.

Then I read on some article I can purchase SIP direct growth fund hdfc amu account but not from HDFC securities.

Now Based on above information below are my queries:

1. Why can’ we purchase SIP direct growth fund from HDFC securities.
2. How hdfc amu is different from hdfc securities.
3. I am already having dmat account on hdfc securities. will it be good idea to create one more account on hdfc amu for SIP investment or I should continue with HDFC securities itself for SIP investment also.

I hope my question make sense. Thanks!