Why Did Takeout Only Export Some of My Albums?
Missing albums are usually shared albums, or the result of selecting specific albums in Takeout. Here is how the album folders actually work.
Usually because shared albums only export the photos you added yourself, because you picked specific albums on the Takeout screen, or because part of an album sits in a zip part you have not unzipped yet.
Shared albums only include the photos you added. Anything a friend or family member contributed belongs to their account, and Takeout will not export another person's data. A shared album from a wedding can come back nearly empty, which is alarming until you know why. Selected albums. If you chose specific albums in the Takeout screen rather than everything, you got exactly what you asked for, and any photo not in a chosen album was left behind. Split parts. Album folders are split across zip parts like everything else. An album can look half-empty simply because the rest of it is in a part you have not unzipped yet.
If you still have gaps, request a fresh export with all photos selected and smaller archive sizes.
Once everything is out, the album folders are just folders, and the photos inside them have no dates. The app reads the .json sidecars across your whole export at once and writes the real capture date and GPS into each photo. It can also sort the results into year and month folders, so your library has a structure that survives leaving Google. The first 500 photos are free.
To download one album from the website instead, see download a Google Photos album. For the whole library, how to download all your Google Photos.