GoKawiilBefore you set up any of Screen Time's modes, you should set which apps will stay available during restrictions. Select Always Allowed to pick from your list of installed apps. Then using the Contacts dropdown will let you select whether you can talk to any phone number or only a list of your choosing.
Downtime is the first option for limiting your usage. When enabled, you can only access the apps you've selected above and phone calls from the contacts you've set. Toggle it manually here, or use Scheduled to customize when it activates on a per-day basis. You'll get a warning five minutes before Downtime activates.
App Limits are the other major way to cut down on phone usage. Choose Add Limit to see a breakdown of your apps by category, like Social, Games and Entertainment. Use the bubbles on the left side to apply a limit to all apps in a group, or expand the dropdown to pick them individually. There's also a Websites button if you want to go beyond apps.
Next, choose the maximum amount of time you want to spend on your selected app(s) each day. As with Downtime, Customize Days helps for setting a different schedule on weekends. When you get within a few minutes of your daily limit, your phone sends a reminder. Repeat the above steps with various groups of apps if you want different time limits for each of them.
Trying to use any app outside the allowed list when Downtime is on, or opening an app after you've exhausted its time, will show a screen saying you've reached your limit. You can choose Ignore Limit to use it for another minute, 15 minutes, or completely ignore the restriction for today. Consider hiding addictive apps for another layer of separation.
Enable Share Across Devices to have your limits sync across every iPhone, iPad, and Mac using your Apple Account. If this is disabled, you'll need to configure each of your devices individually.