I know Jira should have a time tracking feature turned on by default (and in fact it is turned on in settings) but I don't see nowhere a way to add time spent on an issue on the web version.
Instead, I am able to do that with the mobile app, through a clock icon in the right top of the screen, but I cannot see work logs added via the mobile app in the web interface.
Ok, I found it.
You need to go in project settings, click on the issue type for which you want to enable time tracking, and add the Time Tracking field from the list of available fields on the right column.
Repeat it for other issue types for which you want to have time tracking enabled.