This has been one of the continual strong points of the Raspberry Pi and it is this flexibility in redefining its I/O is why the Raspberry Pi has remained popular.
If you are new to the Raspberry Pi and its GPIO, you are going to feel like you have a large learning curve in front of you. I won’t disagree with you.
The most common interfaces that you will come across with the GPIO are the following
PWM, 1-Wire, I2C, SPI and UART
and the main change in the Raspberry Pi 4 is the number of additional ports available for these interfaces.
Most of the tutorials on this website have been done keeping in mind that they should also work on earlier models using the examples.
I have recently updated (quite a bit) my “poster” for my wall above the workbench, as there as quite a few changes from the Raspberry Pi 2/3 GPIO alternate functions of the pins.
You are welcome to use it as you wish. I could have colour coded it per pin group, but I find it easier to locate what I am after. Click on the image to enlarge it.
I am cleaning up the same charts for the Raspberry Pi 2/3 and will have them online soon.