We're talking about this again? Okay. It's simple.
UI–or User Interface–is like the steering wheel and dashboard in a car. It's the stuff you use to interact with and control a system. Turn the wheel and the car turns. Move a handle and the blinker comes on. Twist a knob and the air conditioning comes on. You get the idea.
With apps and websites it's the buttons, navigation, text, and images that create the interface you use to accomplish tasks. It can also be voice instead of visual. "Alexa, turn on the lights" is just as much interacting with an interface as tapping an icon on your phone.
UX–or User Experience–is the feeling you get from driving that car. The wind in your hair, goosebumps from accelerating, turning, etc. It can also be the feeling of crashing that car into a wall. People have feelings about the products and services they use. The frustration you have with a dull knife not cutting a tomato is the same as rage-clicking on text that looks like a link.
Lots of people have the misconception that UI and UX are the same thing. They're related, sure. But not the same thing, not even siblings. Some people also mistakenly believe creating a beautiful UI creates great UX. Wrong. Just because something is beautiful doesn't mean it's usable or desirable.
"UI is how things look and work. UX is how thing feel when you use them."
Great design starts with an understanding of how to create an experience people will enjoy while accomplishing a task that's important to them.
From simple things like finding a show to watch on Netflix to the life-and-death task of monitoring an ICU patient's vitals. Both tasks happen on a screen, and both need to deliver a good experience to the user.
Don't fall in the trap of thinking your product's UI is the main concern and the UX will automatically be good so long as the UI looks nice. Flip the script. First define the experience you want to create, then build a UI that makes that experience possible.