Does Uninstall remove an app's system helpers or extensions?
Short answer: It removes the app and its per-app files in your Library. System-wide helpers and extensions installed by security tools, VPNs or drivers are best removed with the developer's own uninstaller.
Storage Studio's Uninstall covers the app and its per-app files in your Library. Some apps — security tools, VPNs, drivers — also install system-wide helpers or extensions in protected places, and those aren't part of the list.
For apps like that, use the developer's own uninstaller if it has one; it knows exactly what it installed. You can still use Storage Studio afterwards to sweep up any leftovers in your Library.