The developer.paypal.com is live PayPal account developer portal. There you could manage your live REST APP information, manage your sandbox PayPal accounts and sandbox REST APP information.
The www.sandbox.paypal.com is sandbox PayPal account portal. You need to use your sandbox PayPal account credential to log into the sandbox website.
To create and manage sandbox account you log into developer.paypal.com/developer/accounts/ with your live PayPal account. To create new sandbox account, click 'Create Account', then choose 'Account Type' as Business(Merchant Account) if the sandbox account is used to receive payments, and choose 'Account Type' as Personal(Buyer Account) if the sandbox account is used to send payments. Choose a 'Country / Region' for the sandbox account. Then click 'Create' button After the sandbox account is created, you could find it in the 'Sandbox Accounts' list. Move the cursor over the '...' button on the right side of the sandbox account, then click 'View/Edit Account', you will be able to view the sandbox account email address and password information. You could also change/reset the sandbox account password there.
