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.

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

[mrozekma@etudes-1 ~] % whoami
mrozekma
[mrozekma@etudes-1 ~] % sudo -u nobody zsh
[nobody@etudes-1 ~] % whoami
nobody

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

$ whoami 

This command prints the current user. To change users, we will have to use this command (followed by the user's password):

$ su secondUser
Password:

After entering the correct password, you will be logged in as the specified user (which you can check by rerunning whoami.