On an iPhone, you can only install apps from the Apple App Store without jailbreaking the device. This is due to the strict control Apple imposes on its ecosystem to ensure security and prevent unauthorized modifications. If the file you are trying to install is not from the App Store, then you won't be able to directly add it to your iPhone 6 without jailbreaking it. Jailbreaking your iPhone can open up the system to unauthorized apps, but it also voids warranties, increases security risks, and might lead to instability.
If the file you want to add is something like music, videos, or documents, you can transfer them to your iPhone using Apple-approved methods like iTunes or iCloud. Uploading these files to a cloud storage service and...