Quick answer
Inside one Google account, every calendar under 'My calendars' or 'Other calendars' can share the same view: check its box and it renders in the grid with everything else. A calendar that lives in a second Google account is different, since the owner has to share it or make it public before it can be added with 'Subscribe to calendar' or 'From URL,' and the phone app keeps two accounts separate rather than merging them into one screen. For every calendar in one place no matter which account it came from, merge the feed links into one link with Calfeed and subscribe to that instead.
Google Calendar already combines calendars, as long as they live in the same account. My calendars and Other calendars share one grid, checkbox by checkbox. The friction shows up the moment a calendar lives in someone else's Google account instead.
Combine calendars inside one account
Every calendar in the sidebar, 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.
Bring in a calendar from a different 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, choose Subscribe to calendar, and enter that account's email, or choose From URL if the calendar is already public.
What the phone app actually does
The Google Calendar app on iPhone and Android lets an account add a second Google account from the profile photo menu. That does not merge the two: the app switches between accounts rather than showing both calendars on one screen.
One link instead
For every calendar in one place, no matter how many Google accounts it is spread across, paste each calendar's feed link into Calfeed's merge tool. It reads every source and republishes one combined link, refreshed hourly, up to 10 feeds free and 25 on Pro. Any calendar app can subscribe to that one link. It is a read-only combined feed, not two-way sync: edits made after subscribing stay on the merged calendar, not the originals.