Apple apps such as Photos, iMovie, Safari, Keynote, and QuickTime Player work with many media formats. Some apps prefer specific formats, but most formats work in most apps without additional software. Older or specialized formats might not work as expected if your device, its operating system (macOS, iOS, or iPadOS), or the app you're using to view or play the file doesn't support the file's format.
Install all available software updates for your Mac and software updates for your iPhone or iPad. After updating, if you still don't have an app that supports the media format used by the file, search the internet or App Store for an app that does. You can search by the name of the format, or by the filename extension, which is the letters shown at the end of the file's name, such as .avi, .wmv, or .mkv.
On Mac, you might be able to identify the format by selecting the file in the Finder, then pressing Command-I to open an Info window. The format might appear after the label Kind, such as ”Kind: Matroska Video File.”