How to switch between users on one terminal?

How about using the su command?

$ whoami
user1
$ su - user2
Password:
$ whoami
user2
$ exit
logout

If you want to log in as root, there's no need to specify username:

$ whoami
user1
$ su -
Password:
$ whoami
root
$ exit
logout

Generally, you can use sudo to launch a new shell as the user you want; the -u flag lets you specify the username you want:

$ whoami
user1
$ sudo -u user2 zsh
$ whoami
user2

There are more circuitous ways if you don't have sudo access, like ssh username@localhost, but sudo is probably simplest, provided that it's installed and you have permission to use it.