5V vs. 3V operation

Hello,
There are Arduino clones that that a 5V and a 3V switch. I think this changes the voltage level of the digital output, right? So the digital output could be a 5V or at 3V.

Is that because many components, like wireless modules, use 3.3V? So the digital output on the SPI pins need to be at 3.3V level, not 5V?

Are there other implications to switching between 5V and 3V? How would one use an Arduino Uno for these wireless modules - the Uno doesn’t have a 3V-5V switch.

A bi-directional logic level converter is one option.

https://www.sparkfun.com/products/12009

On a lower voltage the board will probably use less power, especially if a DC-DC converter
is being used. The maximum supported clock speed of the MCU depends on the supply
voltage too. This is why the Mini Pro board is available as a 5V 16MHz board or as
a 3.3V 8MHz board variant.

Is that because many components, like wireless modules, use 3.3V?

Yes.

bigred1212: A bi-directional logic level converter is one option.

You don't really need "bidirectional" unless a pin can change direction. Even then, an open collector design is usually preferable (eg. I2C bus).

3.3V -> Arduino isn't a problem. Arduino will recognize 3.3V as HIGH.

5.0V -> 3.3V can be done with resistor dividers (if there's only a couple of connections) or something like a CD4050 (if there's a lot).