But what if the Arduino Uno was released in 1980 ... No USB-UART (which also means no USB programming), a different pin layout (Arduino shields likely won’t fit) and more I/Os than the ATmegas ...

The Uno also has a limited number of pins, making it difficult to work with when the project requires plenty of input and output components. However, these two areas are where the Arduino Mega shines.

In the first picture you have the schematic as usual and in the second one is how the schematic looks on the Fritzing breadboard layout. From line 2 to ... is used to attach an interrupt on pin 2. In ...