You need to write it. From docs:

You can execute your custom code when certain events occur. Two steps are required:

 1. Define the ^%ZSTART routine, the ^%ZSTOP routine, or both.

• In these routines, you can define subroutines to execute when the certain activities start or stop.
• ^%ZSTART and ^%ZSTOP must be defined in the %SYS namespace, although they can be mapped to a non-default database.

2. Use the Management Portal to configure Caché to invoke the desired subroutines.

Specifically, if you define the routine ^%ZSTART and ^%ZSTOP and you include subroutines with specific names, the system automatically calls these subroutines when the activity is beginning or ending. The subroutine names are as follows:

• SYSTEM — Executed when Caché as a system starts or stops
• LOGIN — Executed when a user performs a login or logout using the %Service_Console or Service_Telnet services.
• JOB — Executed when a JOB begins or ends
• CALLIN: — Executed when an external program begins or completes a CALLIN

For example, when a system starts, the system automatically invokes SYSTEM^%ZSTART, if that is defined and if you have used the Management Portal to enable this subroutine.

SYSTEM^%ZSTART and SYSTEM^%ZSTOP are run with $USERNAME set to $system and $ROLES set to %All. To run your code with a different username, use $SYSTEM.Security.Login() to set the desired name and then continue with your custom code. If you use JOB to launch any additional processes, those processes will inherit the same username (and roles) as the initiating process.

Enabling %ZSTART and %ZSTOP
Once the routines have been designed, developed, compiled, and are ready to be tested, individual entry points may be enabled through the Management Portal. Navigate to the Startup Settings page by selecting System Administration, then Configuration, then Additional Settings, then Startup Settings, and edit the appropriate individual settings:

• SystemStart, SystemHalt
• ProcessStart, ProcessHalt
• JobStart, JobHalt
• CallinStart, CallinHalt

To deactivate one or more of the entry points, use the same procedure but change the value to false.

Yeah, there's not much info.

To create inventory scan call:

set in = ##class(Inventory.Scanner).RunScan("Version 1")
set dbU = ##class(Inventory.DatabaseComponent).%New()
do dbU.Init(in,"C:\InterSystems\Ensemble\mgr\db\CACHE.DAT")
do in.RootComponent.AddComponent(dbU)                 
set sc = in.%Save()
//do in.WriteToFile("c:\Temp\scanVersion"_$tr($zdt($h,8)," :","Z")_".xml")

And to get diff between two versions/times use SQL:

SELECT
  s1.Name,
  s1.scan->EndTimeStamp,
  s2.scan->EndTimeStamp
FROM Inventory.RoutineComponent s1
JOIN Inventory.RoutineComponent s2 ON s1.Name=s2.Name
WHERE s1.scan=1 AND s2.scan=2 AND NOT s1.SHA1Hash=s2.SHA1Hash

it would return items with hashes changed between scan 1 and 2. The same query can be further modified to accept timestamps, etc.

It only matters that the async node is caught up.

Also you need to use SSL.

It all depends on connectivity (bandwidth and lag).

ZSTART and ZSTOP are kind of vague, how does that fit into ccontrol start and ccontrol stop. Is there any setting where this can be set to automatic?

ZSTART and ZSTOP are executed on startup/shut down automatically.

Check Fileserver project for ideas.

You need so specify your base URL as URL property and pass the parameters in a method:

 set sc = ..Adapter.Get(.httpResponse, "key,key2", "value", "value2")

It would send the following request:

GET /baseURL?key=value&key2=value2 HTTP/1.1