Why Does Apple Photos Show the Import Date?
Because the photo has no date of its own, so Photos uses the only one available.
Apple Photos reads the date from inside each file. When that is blank, it falls back on the file's date, and after a download or a copy that is the day it arrived. So the whole import shows as one day.
You can correct a photo by hand with Image, then Adjust Date and Time. That is fine for a handful. For a library it is not.
Fix the files before importing. Takeout JSON Metadata Fixer writes the real dates in, and Photos then builds the timeline correctly by itself. It runs on your own computer and is free for your first 500 photos.