How can i download adobe illustrator cc ?

Here: https://www.adobe.com/products/illustrator/free-trial-download.html

You will need a valid Adobe CC subscription to continue using Illustrator after the free trial is over. The subscription will be automatically activated after the free trial unless you manually cancel it before hand.

A "legal way?" Potentially yes, by going to the Adobe site and paying for a subscription. You will need to check with Adobe Support to find out about compatibility with macOS Big Sur.

Ref: https://www.adobe.com/products/illustrator/pricing-info.html

Buy it from Adobe.

Yes, you simply go to the Adobe web site, and subscribe to that service. There will be a fee to pay, then you will be able to download and use the program. Is that what you needed to know?