Between Gmails accounts this is quite straightforward to achieve.
Let's say you have 2 accounts, account A and account B. Now you want to see the calendars of account B while logged in account A.
1- Go to Google Calendar Settings of account B.
2- Select your calendar you want to share on the left panel. Then add your account A to whom you want to share with. You can select view only or "Make changes to events" if you wish to directly manage the account B events directly from the account A.
3- You will receive an email about the sharing.
4- Go back to your account A. In the Other Calendar section you will now see the shared calendar from account B.
You are done. Below example of the Day view final result with both accounts side by side.
Repeat the same steps for all the calendars you want to see.
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