A photo library looks reassuring when every thumbnail is visible. But a thumbnail is not the same thing as a locally available original, and an account is not a storage format.
Decide what you want to preserve
For a family archive, consider originals, edited versions and the organisation that makes the collection useful. Dates, captions and albums may travel differently when you export them. Make a short list before downloading years of pictures.
Apple provides ways to copy iCloud data and export unmodified originals. Google offers data exports through its download service. Use the current official instructions: menu names and export options can change.
Start with one ordinary month
Choose a small period containing photos, a video and an edited image. Export it into a clearly named folder. Check whether the expected items arrived and whether they open in a normal viewer.
Look at dates and filenames. If you receive separate metadata files, keep them with the export instead of discarding unfamiliar extensions. The first test is where you discover how your chosen service represents a library.
Check the result offline
Once the download is complete, briefly disconnect that computer from the internet and open several files. This is a practical way to distinguish downloaded originals from items that still need fetching. Reconnect when finished.
Do not delete the source library to prove that the export works. Verification should add confidence without creating a new recovery problem.
Keep the archive maintainable
Give each export a date and keep a simple note of what it includes. Store an additional copy separately, and make account recovery part of the same routine. A tidy archive with a repeatable update process is easier to maintain than a huge, unexplained folder named “final backup”.