Q: How can I migrate my mails from gmail and yahoo into my iCloud account.

Create a local "on my Mac" mailbox and store your old emails there.

Then notify senders to send new emails to your new address.

There may be a way you can ask your old email providers to automatically forward your emails to your new address, but each email provider will be different.

With Gmail you can go to the online Gmail site, go to Settings:

Click on Filters and Blocked Addresses.  Then you can enter the iCloud address to forward all of your email to:

If you ultimately want to leave gmail, be sure you enable the:

Mail > View >  √ Reply-to Address 

... field in your outgoing emails and specify where you want future responses to be sent.