Helper app is what enables your iPhone / iPad to be used as remote control, keyboard and mouse for your computer. You can install it on as many computers as you wish, and it supports both macOS and Windows.
Download Remote for Mac / Windows Helper app
This app is distributed via my website and not Apple AppStore / Microsoft Store because both companies have restrictions on what Store apps can do, specifically they don't allow synthesized input events for keyboard and mouse. This is called sandboxing - and you can read here more about it. To elevate some of your concerns Mac app is notarized by Apple, and Windows app is signed with code signing certificate.