How do I recover lost photos from a folder in Photo events on iPhone?

Assuming you mean one Album inside the Folder named "iPhoto Events" that was created when you migrated an iPhoto library to a Photos library, it should behave like any other Album. Unless you remove a photo from the Album explicitly or delete the photo from the library entirely. Note that Photos makes the distinction between "remove" from an Album and "delete" from the library.

If you deleted the photos from the library recently, they may still be in Recently Deleted where you can restore them. After thirty days, they would not be recoverable.

Photos which were only removed from the Album would still be somewhere in the library, but there is no easy way to locate them unless the Album was taken on a known date.

You can see this:

         If you're missing photos or videos in the Photos app - Apple Support

Basically the above says to look in Recently Deleted and in Hidden. Be sure to check your Library thoroughly-- the pictures may not be where you expect, as dates may have changed. You can use the search field with the date or location. And remember, you can sort the Library by date captured or by date added.

If you don't use iCloud Photos, but you have Settings>Apple ID>iCloud Backup turned on, then you may be able to recover data from the last 30 days or so at iCloud.com, but you would need to look at the date of the backup to be sure it's from before you lost the images. You should look here:

         View and manage iCloud device backups - Apple Support

This requires you to erase everything on your phone and replace it with the backup.

If you don't keep your own backups to an external drive, then there really isn't any place else to look.