Camelot seems to think that you have a Mac, but your signature indicates you have an iPad. And only an iPhone can use iOS 18.3.1, so all this is pretty confusing. Macs and iPads have helpful sidebars in Photos, since they have more screen room.
For an iPhone, the Duplicates view is near the very bottom of the Photos screen:

And, by the way, "Customize & Reorder" allows you to move these sections around, so if you move Utilities, remember where you put it!
There are other apps that will search for duplicates, but many of them are, in fact, sketchy-- and it's hard to know which ones. PhotoSweeper is one I've used on the Mac with good results. But check out the one that's already in Photos on your phone.
You say you want iCloud Photos-- is that right? iCloud Photos copies all your pictures to iCloud.com so that you can see them with a web browser on any computer. It also allows you to save space on your phone. You turn it on in Settings>AppleID>iCloud. But you should buy space there, first-- it's $3/month for 200 GB.
Check back in if you have more questions. (If you have questions about different issues, start a new thread.)