You can specify a pipeline as a set of jobs when creating it. Multiple jobs can be run as part of a pipeline while it runs. A parallel task that runs on an agent consumes each running job. When your organisation doesn't have enough parallel jobs, the jobs are queued and run one by one.
You can run parallel jobs in Azure Pipelines on either Microsoft-hosted or self-hosted infrastructure. You can run a single job at a time in your company with each parallel job. If you're using an on-premises server, there's no need to pay for parallel operations. Only Azure DevOps Services use the concept of parallel jobs.
Use Microsoft-hosted parallel jobs if you wish to run your jobs on Microsoft-managed machines. Microsoft-hosted agents will run your jobs.
Use self-hosted parallel jobs if you want Azure Pipelines to organise your builds and releases but want to run them on your own computers. You'll start by installing our self-hosted agents on your workstations for self-hosted parallel workloads. In your organisation, you can register an unlimited number of these self-hosted agents.