how to make sure your wordpress is clean of any old files from previous themes so you can install a new theme

with no issues from past themes/plugins. The goal here is I am planning to install a premium theme for my existing WP site and use Divi Plugin So far I’ve tried contacting Bluehost but they just did a ‘scan’ or something so I don’t know if they understood what I meant. I’m relatively new to WP. Tinkering for about 2 years. I’ve tried google searching and irrelevant articles pop up. When searching this site I got 0 results. Hope I’m articulating my question well.