Quick answer
Add the Google account to the iPhone itself and its calendar shows up inside the Apple Calendar app automatically: open Settings, find Calendar (grouped under Apps on current iPhones), go to Calendar Accounts, Add Account, choose Google, and sign in. It is a per-device setup, not a merge, since every phone, tablet, or computer that should see both calendars needs the same account added, and Apple Calendar still keeps the two as separate calendars shown side by side. For one calendar that already has both combined before it reaches any device, merge the feeds into one link with Calfeed and subscribe to that instead.
Try it with a real schedule
Calfeed turns it into a calendar link people can subscribe to.
Google Calendar merges calendars freely inside one account and barely at all across two. Getting Google events onto an iPhone alongside iCloud is a third case again, and it needs no app or export, just one account added in Settings. Here is each of the three, and the one link that covers all of them at once.
Merge calendars inside one Google account
Google Calendar already combines calendars that live in the same account. In the sidebar, every entry under My calendars and Other calendars checks on or off independently. Check the ones to include and they render together in the day, week, or month grid, each in its own color. No merge step, no export.
Pull in a calendar from a second Google account
A calendar owned by a different Google account needs permission first. Ask the owner to share it with the account, or to make it public. Then select the plus next to Other calendars and choose Subscribe to calendar, entering that account's email, or From URL if the calendar is already public.
The phone app does not close this gap. Google Calendar on iPhone and Android lets an account add a second Google account from the profile photo menu, but it switches between accounts rather than showing both on one screen.
Add the Google account to an iPhone
In Settings, find Calendar (it sits under Apps first on current iPhones, so Settings, Apps, Calendar; older versions show Calendar directly). Select Calendar Accounts, then Add Account, choose Google, and sign in. Turn off Mail and Contacts there if only the calendar matters.
Check both calendars
Open the Calendar app and tap Calendars at the bottom. Make sure both the iCloud calendar and the newly added Google calendar are checked. Events from both now show up together in the day, week, and month views.
The limit: per device, not merged
This has to happen again on every device that should see both calendars, a second iPhone, an iPad, a Mac. And even on one device, Apple Calendar keeps the two calendars separate underneath. It displays them together; it does not combine them into one calendar.
One link instead
For one calendar that is already combined before it reaches a device, paste the Google calendar's feed link and the iCloud calendar's feed link into Calfeed's merge tool. One combined link comes back, refreshed hourly, up to 10 feeds per calendar. Any device subscribes to that single link instead of adding two accounts. It is a read-only merge, not the same as the native Google account sync: edits made after subscribing stay on the merged calendar, not the original Google or iCloud one. Downstream, Apple checks about once an hour, Google every 12 to 24 hours.