You have the official installer. Now what?
If you have a current Creative Cloud subscription (Individual, Teams, or Enterprise), you can legally download and install older versions of Illustrator, including CC 2015. Here is the step-by-step process (verified as of 2025): download adobe illustrator cc 2015
Before installing, ensure your machine can handle the 2015 architecture: Intel Pentium 4 or AMD Athlon 64. You have the official installer