How WhatsApp Transfer Chat Works...

If you're switching from one Android phone to another, WhatsApp offers a convenient Chat Transfer feature to transfer your chat history and media. While restoring from a backup is an option to restore chat data, Chat Transfer goes beyond transferring all your account information, including personal messages, photos, videos, documents, and more. Please note that this feature cannot transmit peer-to-peer payment messages and call history.

You Must Need

• Android phones
• Android OS Lollipop 5.1
• SDK 23+
• Use the same phone number as your old phone on your new phone
• First start the migration on your old phone then new phone must not be registered on WhatsApp
• Your phones must be close to each other.
• Both phones must have enough battery
• Both phone's WiFi should be enabled 
• The location should be on.

Transfer your chat history 

Old phone

1. Open WhatsApp on your old phone.
2. Tap More options (three vertical dots) > Settings > Chats > Transfer chats > Start.
3. Enable the required permissions and get ready to scan the QR code that appears on your new phone.

New mobile

1.  Download and open WhatsApp on your new phone.
2. Complete the setup process.
3. Tap "Accept Terms & Conditions" and verify your phone number.
4. Select "Start" to transfer the chat history from the old phone.
5. When prompted, grant the necessary permissions to your new phone and a QR code will appear.

Back to old phone:

1. Scan the QR code displayed on your new phone using the WhatsApp camera on your old phone.
2. Accept the connection invitation on your new phone and pair it with your old phone.
3. Keep both phones unlocked and do not exit WhatsApp during the transfer process. Messaging will be suspended while the chat history is being transferred. You can cancel the process at any time if needed.

After conversion:

1. When the import is complete, you will be notified that the process is complete.
2. Click "Done" to complete the chat history transfer. Now your old WhatsApp chats should be available on your new phone. 

Content reference is How to transfer your chat history