Digital Wellbeing's features are being heavily emphasized since Android Pie. During I/O 2019, Google announced several enhancements to the embedded service including parental controls (which are already live), per-app grayscale settings (not yet), and Focus mode. The latter is now available in the latest Digital Wellbeing beta v.1.0.263.
After updating the app to the correct version, you need to force stop it then launch it to get Focus mode to show up. Tap it then select all the apps you want to pause when you're working. Your most used apps are on top, so it should be easy to select the ones you usually lose a lot of time on.
Focus mode in Digital Wellbeing.
After selecting your distracting apps, you can start Focus mode from the button in that screen. You can also add a Quick Settings toggle to speed up the process in the future.
Adding the Quick Settings toggle for Focus mode.
When the mode is on, all your time-consuming apps will be greyed out in the launcher and tapping them shows that they're paused (similar to when you manually pause an app). If you really want to open one of them, you have to go into settings to change that app's permission. Also, all notifications from these paused apps will be hidden. They'll resurface the moment you stop Focus mode.
Greyed out paused apps.
Used in conjunction with Do Not Disturb, Focus mode can be helpful when you need to dedicate all your attention to your studies or work, while also staying reachable. However, there's nothing stopping you from using it the other way around to disable all work apps and communications in your off-hours or during vacations.