Jenkinsfile equivalent in Azure repos
I'm new to Azure DevOps so I may be wrong - I think you are asking about using a Jenkins plugin to scan Azure Repos for Jenkinsfiles.

If you were moving to Azure DevOps, you can store your Azure Pipeline configuration, azure-pipelines.yml,  in YAML format and this can be stored in the code repository (like a Jenkinsfile).

For further pipeline details see; Create your first pipeline and YAML schema reference.

It looks like you can configure Azure Pipelines to scan your organization repos in Azure Repos and GitHub

The GitHub documentation does warn that this feature is actively being improved - so it may change in the future.

For GitHub you need also configure Azure Pipelines access using the GitHub app:

https://github.com/apps/azure-pipelines/installations/new/permissions