You work on multiple projects (for multiple clients) on multiple browser tabs at the same time.
Time tracking app that lets you assign each browser window to a particular project. This way when you are multitasking (because you are waiting for something to complete on one browser while doing something in another), the app will automatically track all your work time on all projects simultaneously.
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.