Can I downgrade my iPhone from iOS 18 to the previous version?

Nope, there is no supported way to go back to iOS 17. For specific changes for such things as Control Center, it is now completely customizable, If you have not used any of the new features to customize the look, you should start here:

Use and customize Control Center on iPhone - Apple Support

Photos app is also now completely customizable. In addition to changing the size of the thumbnails on your screen, you can also review the steps here to customize it along with additional information on viewing your photos. If you do not like the Photos app, there are many other options available in the App Store, so the choice is yours there which one would meet your needs.

Customize the Photos app on iPhone - Apple Support

You now see bigger emojis because users had previously complained they were too small and send feedback to Apple. You can do the same and maybe they can make the size adjustable. In any case, Apple won't see your post here, so the place to do that is with this link:

Product Feedback - Apple

You can expect a change in iOS every year as that has been the standard upgrade schedule. If Apple never made changes that added new features to iOS, users would complain that Apple is not being innovative, and when changes are made, then users complain they didn't want any changes. We saw the same complaints when for years there were no changes to the Calculator app as users demanded more features. It really is a no win in either case, and would recommend that if you don't want to see any changes, then make sure to not update when you see the new version available.