I've read some info:
Still fuzzy about those concepts.
First off, these three links should give you as much information as you would need to understand each of these.
Start-Process
Start-Job
Start-Service
Depending on what you want to accomplish, you can chose the necessary cmdlet.
Start-Process
runs an executable on the system (Notepad, powershell, Sort.exe etc with arguments if needed)
Start-Job
runs processes in the background for asynchronoush / parallel jobs
and Start-Service
starts a windows service (like systemd) "already" configured on the server. This does not create or configure the service, simply starts it.
Each of the links above provides examples on how to use them as well.