When adding a google account to you iPhone, sometimes not all your calendars will display. Or if you decide to add some delegates (other calendars, .ics formats,…) they don’t show up on your phone to activate them.

Google has a setting to enable the calendars you want to see on your phone.

This can be found over here: https://calendar.google.com/calendar/syncselect

I have no clue how you can acces this page through a menu, but after some googling you can find the direct link to this page.