How Do I Add Location to My Photos?
If the coordinates exist somewhere, a tool can write them into the photo. If they were never recorded, they cannot be invented.
Coordinates go into the photo's GPS fields. If your photos came from Google Takeout, the coordinates are already in the .json files beside them and only need copying across. That is the common case and it is quick.
If the photos never had a location, nothing can recover one. Some tools let you place photos on a map by hand, which is fine for a holiday and hopeless for a library.
Takeout JSON Metadata Fixer writes the coordinates from your export into each photo, and can look up the place name too, so you get Paris rather than a pair of numbers. It runs on your own computer and is free for your first 500 photos.