1. For Customer login:

Use core to test:

Go to file:
Magento\Customer\Controller\Account\LoginPost.php
Add this code behind function execute().

$this->session->loginById($customerId);
$this->session->regenerateId();

$customerId is your customer id would like to login.

Use plugin

Create di.xml:

Create file plugin:

Also, you can use another way as :

• event: capture event “controller_action_predispatch_customer_account_loginpost"
• rewrite: override function execute via file:
  

  Magento\Customer\Controller\Account\LoginPost

2. For Admin login:

Use core to test

Go to file:
Magento\Backend\Controller\Adminhtml\Auth\Login.php
Add this code behind function execute().

Another way same customer:

• Plugin: Use plugin for class \Magento\Backend\Model\Auth\Session.php with function isLoggedIn()
• Event: controller_action_predispatch_adminhtml_auth_login
• Rewrite: Class Magento\Backend\Controller\Adminhtml\Auth\Login.php function execute().