You can open chats in a separate window in Teams using several ways.

Option 1: Double-click on the chat you want to pop out

Option 2: Select the chat you want to pop out > click on the pop out button

Option 3: On your current chat window, click on the pop out button on the right

Option 4: Right-click on the chat you want to pop out > select Pop out chat

Option 5: You can use the command /pop then search for a user. A pop out window will open when you click on the user