Most Apps document this e.g. https://help.plobalapps.com/en/articles/1829669-how-to-uninstall-plobal-apps-from-shopify-app-store
If it isn't documented then I'd recommend you contact the App vendors directly.
Hyde here from Shopify.
Sometimes if you delete an app it leaves behind residual code in the theme that needs to be removed.
As it is a third-party app, we cannot delete that code for you. You would need to contact the app developers and ask them to help remove the code.
I hope that helps! Happy to answer any other questions you may have.
All the best, Hyde.