How do I intergate more than one calendar?

What I did: signed in with my Apple ID

What happened: I can only see my calendar attached to that apple ID

What I expected:An option to add multiple calendars

Things that might be helpful to know (Agenda version, OS and model, etc): I am using a MacBook Pro, Iphone 13 pro max, and I would like to add my google calendar. [not my apple calendar]