Photos in the Library are sorted by date. You can adjust the date of photos by opening them, then tapping on the (i) icon at the top right. I don't recommend this though. It makes in impossible to find them by date later.
Instead, you should create albums for the photos you want to keep together. Inside an album you can drag photos into any order you like. Also, you can have the same photo in multiple albums, in case they fit more than one category; like maybe both Christmas and Family.
It doesn't duplicate the photo. It's just a very flexible way to organize your photos, in to as many different ways as you're willing to spend time doing so.
To help with creating albums from a large photo collection you can use the People, Places, and Search to find the photos you want to add to the ablum. You'll probably miss a few that way (I always do) but it helps gets the bulk of them where you want them to be.
Note: Changing the photos date has only been available on the last few versions of iOS/iPadOS, however there are utility apps that will let you change dates and a few other bits of metadata. I've used the app Metapho for this int he past. ViewExif is another good metadata app. It doesn't allow editing like Metpho, but shows more data.