SCCM Site Boundary

See https://docs.microsoft.com/en-us/sccm/core/servers/deploy/configure/define-site-boundaries-and-boundary-groups

Boundaries in Configuration Manager define network locations on your intranet. These locations include devices that you want to manage. Boundary groups are logical groups of boundaries that you configure.

This has nothing to do with your Active Directory structure.

Usually you install at least a distribuition point in each site and configure the boundaries so each machine on a site will accesses the local distribuition point. This saves a lot of bandwidth between your sites.

the link above states:

A hierarchy can include any number of boundary groups. Each boundary group can contain any combination of the following boundary types:

IP subnet
Active Directory site name
IPv6 prefix
IP address range

So if you have so many subnets  in different geographical locations, but you have only one Active Director site, then you  still can  set up one SCCM Site Boundary .. Correct ?

Yes, exactly. The boundary is the IP subnet

Active Directory Site cannot be a Site Boundary  ?

the link above you posted says you can...

In the Scenario where We have only one Active Directory SIte, even though we have several physical site

Address the problem. Create sites for each physical location and assign the correct subnet to them whether you have DC there or not. It is best practices and you will be able to use this within SCCM, DFS, GPOs etc.

Thank you