Returning active ID vs execution's ID

what is the point of the active ID for workflows? why is active ID not an attribute on the Mongo execution object?

I am asking because I noticed that all the /executions endpoints in Server.ts query execution objects by ID in the database, but how do you get the execution ID in the first place? It seems like /run is returning the active ID only, when it would be more useful to receive the execution’s database ID and have active as an attribute on it

The active execution ID is for an execution that is currently active. So it is currently running. Every execution gets one and it exists only in the memory and only as long as the execution is active. It is needed to be able to identify and manually stop a currently running execution.
The execution ID is the ID of a past execution that got saved to the database. Not all executions get saved to the database. If it gets saved there, depends on the global settings, the workflow-settings and on if the execution was successful or if it did error. Here the settings:
https://docs.n8n.io/#/workflow?id=workflow-settings
https://docs.n8n.io/#/configuration?id=execution-data-errorsuccess

ah that makes sense, so then where are we notified when execution is actually successful / saved?

There is currently nothing that does that as it was never needed (at least not until now). If something has to get informed about a workflow being finished, the workflow normally takes care of that itself. The last node would then, for example, be an HTTP-Request Node which sends information somewhere.