Are there any USB flash drives or SD cards which use RAID or redundant storage for additional reliability

Yes. Their behavior is identical. Essentially is the same thing.

Files aren't physically sorted in the way you "see" them: files are not located "inside" folders. In fact the're all around the drive. Then you have a folders and that folder is a file which list the locations of the files "inside the folder". When you delete a file, it's deleted from it's location in that file, physically the file remains stored.

Think in a book: the index says where the page is located. If you remove an entry from the index page that chapter doesn't get removed from the book.

So, data deleted can be recovered.

In another hand you're almost right: after deleting a file stored in a traditional magnetic storage it can be recovered at least it's overwritten. After several write passes it can't be recovered. Think in a notebook: when you write the pen prints the shape to the sheet below, and the another below that, etc. That's a traditional magnetic storage. Your question refers to flash memory which doesn't work so. After one pass overwritten there's no data recoverable; so if you delete and overwrite the entire disk you won't need seven passes to secure it.

Now getting more complex: since flash memory are limited in the number of writes they have algorithms to avoid overwrite a once and again over the same piece. They write where have less writes accumulated.

Final and simple recommendation: Format the device then get it full loaded with mp3 or video, then format again. zero index available after that.

Parallelism, and how the controller makes use of it is the main factor. There usually isn't room for the 8-16 discrete NAND chips you'd find in an SSD. The controllers in USB stick usually aren't nearly as complex either, to efficiently make use of available parallelism.

Other factors that matter are the quality of the flash. Many USB sticks use cheaper flash which you have to read and write slower in order to avoid errors. USB drives don't have lots of over-provisioning to help keep clean blocks around. And USB 2.0 is limited to around 35 MB/s. And many SSDs come with a big RAM cache.

The main reason for these differences is price. There are some SSDs in a USB stick form factor, like the LaCie FastKey.