How do I split large audio/video files into 30-minute segments on MacBook Air?

The best tool for this 30-minute segment approach is ffmpeg run from the folder containing the large A/V file:

ffmpeg -i foo.mp4 -c:v copy -c:a copy -f segment -segment_time 00:30:00 -reset_timestamps 1 -segment_start_number 1 -map 0 output_%03d.mp4

The ffmpeg tool can be built from sources by you, or via homebrew package manager. If you build it (below), you won't have to deal with the 43 dependencies that homebrew must install during the build process. Those dependencies will require updating in the future as new versions of each are released. That is a black hole.

Build it yourself from source code. Reasonably automated.

1. Install Command Line Tools for Xcode from Terminal
   1. xcode-select --install
   2. Proceed with the dialog that appears. The installation will consume ~ 4GB.
2. Download the current ffmpeg build (right now it is 8.0.1 (Huffman) as [ Download XZ tarball ].
   1. From Finder and in your Downloads folder, double-click ffmpeg-8.0.1.tar.xz to expand it into the ffmpeg-8.0.1 folder
   2. In the Terminal, cd ~/Downloads/ffmpeg-8.0.1
   3. ./configure
   4. make
      1. Ignore any warnings.
   5. make install. This puts ffmpeg into your /usr/local/bin folder.
      1. It also puts the ffmpeg man pages in /usr/local/share/man/man1
      2. man ffmpeg
      3. ffmpeg --help
   6. make clean
3. Place your large A/V file in a folder by itself. Launch the Terminal and change directory to that folder location. Use the ffmpeg command sequence I provided above and change foo.mp4 to your A/V filename, escaping any whitespace in the filename if it exists. Then press enter to run ffmpeg. It is reasonably quick and will spew considerably in the Terminal window. Your split video files will now be in that folder.

In the Terminal and within the folder where you split the A/V video, you can loop through each split video file to see its duration in seconds:

for f in output_*.mp4;
do
   mdls -raw -name kMDItemDurationSeconds "${f}" | xargs
done

Audacity for audio files.

Maybe iMovie for video?

I cannot.

It's been quite a while since I used iMovie to do video editing. But you might find this useful: iMovie User Guide for Mac - Apple Support

Audacity is pretty straightforward as editors go. You can find good guidance if you google it. You can also visit the Audacity website for additional help. https://www.audacityteam.org/