Here is a table that is what I was looking for, note that I'm going to change your use of /a to represent the agent working directory and instead anchor it to / noting that it technically could be anywhere.
Directory
Uses
Variable
Task Magic
/
This is the working director for agent
Agent.WorkFolder
/1
This is the working director for agent
Agent.BuildDirectory
Dowloaded Artifacts Are placed here
/1/a
Artifact staging directory. This is where the VS Build task results are stored in. Note - it gets purged before each new build.
The one I needed was: Agent.BuildDirectory
https://docs.microsoft.com/en-us/azure/devops/pipelines/build/variables?view=azure-devops&tabs=yaml
Here is a table that is what I was looking for, note that I'm going to change your use of /a to represent the agent working directory and instead anchor it to / noting that it technically could be anywhere.