How to aggregate logs from several Jenkins Jobs\Pipelines in one place?

You can reference this doc to find where the default logging path is for Jenkins depending on your OS. (For linux, it's /var/log/jenkins/jenkins.log if you don't configure it to be something else.

Then as long as your Datadog agent is v6+ you can use the Datadog agent to tail your jenkins.log file by following this doc.

Specifically, you'd add this line to your dadatod.yaml:

logs_enabled: true

and add this content to any old conf.yaml file nested in your conf.d/ directory, such as conf.d/jenkins.d/conf.yaml:

logs:
  - type: file
    path: /var/log/jenkins/jenkins.log
    service: jenkins
    source: jenkins

Then the agent will tail your log file as it's written to, and will forward it to your Datadog account so you can query, graph, and monitor on your log data there.

Once you have the logs coming in, you may want to write a processing pipeline to get the critical attributes parsed out, but that would be material for a new question :) .