iCloud mail folders not showing up on Mac M1 Mail

Sign in with the same Apple ID & password say example1@icloud.com on your iPhone & Mac and connect both with an usb cable .Overview of syncing your Mac and your devices - Apple Support

Click on finder > general > sync 

Let the bar be completely loaded .

Open your mailbox in iPhone 

Click on edit option on the up-right corner 

Click on “new mailbox” option on your down-right corner ,

In the new mail box , in the name section enter your iCloud id say example1@icloud.com

And set your mailbox location as per you requirement (iCloud / gmail) - prefer iCloud 

Then again click on edit , you will see your apple id , under it you will find an option of “new mailbox '’on your right corner > chose the name for your folder and add the mailbox location ( iCloud id )> click on save > finally click on done .

The folder is created in iPhone , from web browser ( mac ) open www.icoud.com in mail section in the bottom left corner click on refresh , the mail folders will be visible after few moments , then open IMAP account in mac  Add or remove email accounts in Mail on Mac - Apple Support under iCloud section in the bottom click on drop down arrows all mail folders will appear .