How do hackers steal a Google account or any other cookie by actually copying the victims cookie?
This is how cookies work in general:
A user's session cookie acts as their credential after they've been authenticated and until they are signed off. A user provides valid username/password and gets a session cookie in return. Its this cookie that each subsequent HTTP request needs to present to the server so that it knows the 'context' of the request. This ensures that a user isn't asked for the username/password for each page/request.
Hence, if you obtain the session cookie of a logged in user, you can essentially impersonate him/her without actually knowing their password. A very common method to steal cookies are our beloved XSS attacks. However, a good combination of CSP settings and protections like XSS auditor combined with httpOnly cookies thwart most XSS attacks. If you find one on gmail, you'll probably get a hefty sum of $$ via their bug bounty program ;)
CSRF is another way how attackers make servers perform changes that weren't authorized or initiated by the user. There was a cute CSRF attack on gmail where a attacker could send POST requests to create an email filter on user's account which will essentially forward a copy all emails to an email of attackers choice. Read about it at: https://www.wired.com/2007/09/gmail_flaw_lets_anyone_read_your_e-mail/
Obviously there are bunch of other attacks that can be leveraged to compromise user's account or data in some way or the other.