Download the WordPress theme from ThemeForest
Right after you completed the purchasing a WordPress theme on ThemForest, a download button will appear. Just click on that button to download the theme.
In case you want to download the theme again or somehow you closed the browser tab, you need log into your account on ThemeForest. Then move your mouse over your username on the top right and click Downloads.
Then you will be redirected to your Downloads page. All your purchased items are listed here.
Each WordPress has a green button Download. Clicking on that button shows a dropdown. Then select All files & documentation to download the WordPress theme and everything goes with it that you might need to set up the theme, such as required plugins, documentation.