You may want to check the Settings for the Camera on the iPhone. The Camera may be saving the photos and videos in the space saving High efficiency format, but a Mac on el Capitan cannot read this format. You need at least High Sierra.
Either set the camera on the ihone to save the photos in a more compatible format, of change the settings for the transfer to a computer so the files will be converted to a formt that the Mac can read.
See: Using HEIF or HEVC media on Apple devices - Apple Support
To take photos in a more compatible format:
Though capturing in HEIF and HEVC format is recommended, you can capture media using older formats that are more broadly compatible with other operating systems and devices:
- In iOS 11, go to Settings > Camera.
- Tap Formats.
- Tap Most Compatible. This setting is available only on devices that can capture media in HEIF or HEVC format.
- All new photos and videos will now use JPEG or H.264 format. To return to using the space-saving HEIF and HEVC formats, choose High Efficiency.
To transfer as JPEG or H.264:
When you import HEIF or HEVC media from an attached iOS device to Photos, Image Capture, or a PC, the media might be converted to JPEG or H.264.
You can change this import behavior in iOS 11. Go to Settings > Photos. In the "Transfer to Mac or PC" section, tap Keep Originals to prevent the media from being converted when importing.