I believe technically both UDP and TCP are implemented on top of IP.
But UDP is basically IP + a port number and a checksum, while TCP is IP + a port number and includes a checksum as well, and a bunch of other stuff. So while TCP isn’t exactly implemented on top of UDP, it’s pretty close.
Something very much like TCP can be implemented on top of UDP, with whatever improvements or differences you might want to implement. (It’s just that both sides need to understand the custom TCP-like protocol.)
FWIW the properties of UDP can be provided by a custom protocol implemented to look like it's TCP. Send acknowledgements for packets that were actually dropped, etc.
I've seen a few pieces of niche Free software that do such a thing to get around super restrictive firewalls.
But UDP is basically IP + a port number and a checksum, while TCP is IP + a port number and includes a checksum as well, and a bunch of other stuff. So while TCP isn’t exactly implemented on top of UDP, it’s pretty close.
Something very much like TCP can be implemented on top of UDP, with whatever improvements or differences you might want to implement. (It’s just that both sides need to understand the custom TCP-like protocol.)