Configuring a service hook in Azure DevOps for Jenkins CI you have to choose an action — Trigger generic build or Trigger Git build.
Which one to choose?
Trigger generic build
This one forces you to choose a build. For a multibranch pipeline, it would be a combination of Jenkins job and branch, like <job-name>/master.
This is ok for freestyle job with only one branch for CI.
Trigger Git build
If you are making CI for features, fixes and pull-requests this action is recommended one. Jenkins Git plugin with 'Discover branches' behaviour in a job takes care of the rest.
It's enough to make a single service hook with this action for all repositories in a project.