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It is the only simulation that I use, because it is the only simulation that works. It has limitations, but it is fun to try small things.
An old slow computer might have troubles, because the simulation needs a fast browser/computer.
Click on the left: Circuits
Make a new circuit.
Give it a better name in the upper-left corner.
Drag the Arduino Uno from the right into the workspace.
Click "Blocks" and select "Text" for a normal sketch.
You will see a sketch to blink a led. Start the simulation and check if you see the 'L' led blinking.