Google — Click the Google button and confirm when prompted. Google will name the calendar after its URL; to rename it, click the three dots next to it in the sidebar → Settings → Name. The calendar syncs automatically every ~24 hours. On mobile, make sure the sync toggle is enabled for the calendar in the Google Calendar app settings.
Apple — Click the Apple button; it opens directly in Apple Calendar. Confirm when prompted. The calendar refreshes automatically based on your system's fetch interval (Calendar → Settings → Accounts → Fetch New Data).
Outlook — Click the Outlook button; it opens with the calendar pre-filled. Confirm when prompted. Outlook refreshes subscribed calendars automatically every few hours.
Proton Calendar — Click Copy link, then in Proton Calendar go to Settings → Calendars → Add calendar → Subscribe to calendar and paste the link. Proton refreshes subscribed calendars automatically.
Other apps — Click Copy link and paste the URL into your app's "subscribe" or "add by URL" feature. Most calendar apps refresh subscriptions automatically.