Many times you’re doing things in parallel. If you want to do your best you should really only concentrate on one thing at a time. There are, however, many tasks that can be done in parallel. Buying groceries and listening to an audiobook or a lecture. Doing 2 or 3 maintenance chores on your computer, switching between them when there’s a wait etc.
Have an online app that can track many tasks in parallel. You can start one task and a timer for it, then when you have to wait and switch to a new task you can start another timer for that as well. Both timers running in parallel until you stop them.