Azure implementation AAD connect

Copper Contributor
Hi,
Building a new MS AD domain on server 2019 for the office. Next to the MS AD domain we have Azure AD which is leading and where the tenant from office 365 is managed.
I would like to connect al my office users and their machines to the same Azure AD, implement SSO for some older applications and hardware.
A.
Can i sync from Azure AD to MSFT AD on premise?
B.
Do i need AADDS in Azure?
C.
What kind of redundancy do i need to have when for example the Azure connection is failing?
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