Find and merge duplicates - create leads from third party app

Hi Community! Hope you are doing well.

My goal is to create leads from third party app, obviously avoiding loading duplicates.

I have those questions:

1. I know that it exists in the process element "Find and Merge duplicate". What is the result of this item? Does it tell us what is the ID of the record that remains? The new record created or the merged record in which result/parameter? gOldRecordId?
2. I'm working on a source code schema. Can I re use some code to achive my goal? I was reviewing some other services that create leads from landing pages or others, but they are not simple to adapt. Is there any method that allows me to validate duplicates or to do the merge by code after having saved a lead?

Any other comment or suggestion are welcome.

Thank you!

Regards.