Yes Aseprite is a paid Pixel art software (only on PC) but you can also get it free legally.
There are two ways you can get Aseprite.
(1) To purchase it online. Visit www.aseprite.org or purchase it on Steam or Itch.io
(2) You can Compile Aseprite on your computer from their official GitHub repository for free . This is 100% legal. If you are good with code and you know your way around it , it would be easy for you.
Search for their GitHub repository on Google or simply watch some YouTube tutorials on it. It will guide you.
Hope this helps 😁
RE: Dog Pixel ART timelapse Tutorial using Aseprite