If you received an .ics or webcal subscription link:
The calendar will update automatically when its organizer changes events. Only subscribe to calendars from sources you trust.
The Google Calendar Android app cannot directly add a new calendar-subscription URL. Use a web browser:
.ics or webcal address.It will then update automatically. Google confirms that new subscriptions must be added through a web browser, not through its Android app. Google Calendar Help
On a Samsung phone, a Google calendar added this way should also appear in Samsung Calendar after syncing. Open Samsung Calendar → ☰ → Settings/Manage calendars, enable the Google account, and tap Sync now if needed.
Most ICS subscriptions are read-only. Thunderbird instructions
Outlook subscriptions update automatically, although changes may sometimes take more than 24 hours. Microsoft instructions
Depending on the version, the path may instead be:
File → Account Settings → Account Settings → Internet Calendars → New
Paste the address and select Add.
Look for one of these menu options:
Paste the https://, webcal://, or .ics subscription address when prompted.
Important: Subscribe keeps the calendar updated. Import or opening a downloaded .ics file usually creates only a one-time copy that will not receive future changes.