Why Do My Photos Have the Wrong Year After Importing?
A whole library landing in the wrong year is the import date, not your photos. Here is the cause and the fix.
Because your photo library app used the date it imported the file, not the date the photo was taken.
Every photo app reads the capture date from inside the image. Google Takeout photos often have nothing there, because Google kept that information in its own database and exported it to a separate .json file. Your app looks inside the picture, finds nothing, and uses today. That is why everything lands in one year instead of spread across a decade.
Fix the files before you import them and the problem never happens. The app writes the true capture date into each photo, so your library app reads it and files everything correctly on the first pass. Doing it afterwards means correcting dates by hand or deleting the import and starting again. The first 500 photos are free.