Salesforce orgs other than scratch orgs do not currently provide source tracking, which makes it possible to pinpoint user changes in metadata and extract only those changes. This is done by an SFDX/Metadata API client, like Salesforce DX or CumulusCI (disclaimer: I'm on the CumulusCI team).
I would not try to implement a Metadata API client in PowerShell; instead, harness one of the existing tools to do so.
Salesforce orgs other than scratch orgs don't provide source tracking at present. To identify user changes, you can either
- Attempt to extract all metadata and diff it against your version control, which is considerably harder than it sounds and is implemented by a variety of commercial DevOps tools for Salesforce (GearSet, Copado, etc).
- Have the user manually add components to a Change Set or Unmanaged Package, and use a Metadata API client as above to retrieve the contents of that package. (Little-known fact, a Change Set can be retrieved as a package!)
To emphasize: DevOps on Salesforce does not work like other platforms. Working on the Metadata API requires a fair amount of time investment and specialization. Harness the existing work of the Salesforce community where you can, but be aware that the task you are laying out may be rather more involved than you think and it's not necessarily something you can just throw together from off-the-shelf components.