1. Interfacing different devices & considerations

2. Compatibility is the first thing to be considered when devices need to be interfaced Voltage Current Impedance & Rise time Direction

3. Voltage
A circuit can be designed to respond when something exceeds a threshold based on either current flow or voltage drop/gain.
Say, an IR sensor pair. The current flow through the photodiode in reverse biased is limited to some uA. But when it receives some light, the current flow gets in the range of some hundreds of mA.
Based on the voltage drop across the resistor connected in series to photodiode, something else can be done. Higher the series resistance, stronger the logic level. So, voltage drop is a consideration.

4. Higher the series resistance, stronger the logic level.

5. Current
Similarly, a microcontroller can’t drive 12V DC geared or a 5V Stepper motor. As microcontrollers are limited to tens of mA & current required by motors is some hundreds of mA, a device that makes a draw as required is necessary & a driver gets it done. So, current is a consideration.

6. Impedance & Rise time
Impedance is a certain consideration when things like switches, sensors, keypads need to be interfaced.
If this pull down resistance is some tens of mA, the source meant to serve as supply gets ruined as high current flows exceeding the maximum rating.

7. So, proper resistances need to be selected so that the supply cannot be ruined. But, there is an issue with higher series resistance as combination of resistors & capacitors increase the time constant making the processing delay more.
A resistance & capacitor make a time constant for a circuit loop.

8. Higher impedance
Lesser impedance

9. Direction
Setting the direction of a port/pin depends up on the purpose. When a device has been made to capture something, the pins/port where the external device has been interfaced should be configured as inputs.
Ex: switches, sensors, pots analogRead(A4, INPUT)
When something has to be driven or controlled, the pins/port should obviously be configured as output.
Ex: LED, Motor analogWrite(A4, INPUT)