What Should I Do First After Downloading Google Takeout?
The first hour decides how painful the rest is. Here are the four steps to take before you import anything anywhere.
Four things, in this order. Getting them right saves you from redoing everything later.
One: unzip every part into a single folder. Not one folder per part. A photo and its .json sidecar frequently land in different archives, and they have to be together to be matched. Two: do not delete anything from Google Photos yet. An export is not a backup until you have opened it and counted it. Three: do not import into Apple Photos, Immich, Nextcloud or anything else yet. Your photos currently have no dates inside them, and importing broken files means fixing them twice. Four: check your computer's timezone, because the sidecar timestamps are UTC and get converted using your machine's setting.
Then, and only then, fix the metadata.
That is the step that turns a folder of files into a photo library. The app reads every .json sidecar, writes the real capture date, time and GPS into each photo, sets the file timestamps to match, and can sort everything into year and month folders on the way out. After that, import wherever you like and it will simply work. Everything runs on your own computer, and the first 500 photos are free.