User activity is logged for each process so you can always see who did what and when. Activity log is available for every task and process.
For any process you can find it on the right panel -"History" tab
History section displays all process events, including its date and time and event description.
The list includes events like:
Task becomes available for work
If you need to see events related to a specific task, you can open the task view page and find "History" tab: