How to use radios?
 

I know recent airliners have as many as three radio sets.
But only three!
One is always used for ACARS.
One is for ATC communication.
The other is for 121.5Mhz?

If so, how about company radio or ATIS?

Then I'm wondering if 121.5 should be monitored by every pilot at any time?
I do not think old single piston airplanes have only one radio, which should be used for ATC.

Thank you for any input!

Temporarily switch frequency on one of the non-ATC radios.

Yes, MANY old airplanes had only one - or NO - radio!

121.5 doesn't need to be continuously monitored.
Not during engine start, taxi and all that other good stuff.
You want to monitor 121.5 inflight in case you cock up a frequency change and ATC is trying to reach you.
When nothing else is going on you monitor for somebody else's problems.
But international crews might be jabbering on all three radios at once.
Two FIR crossings with no handover and the third to keep the current controller happy.
Radios are tools, use them.

One for ATC, second set on the ground for the OPS frequency, in the air to 121.5, third is fixed on ACARS. ATIS? ACARS if possible, if not one pilot switches on the second set to the ATIS frequency and copies the information, except if you fly for ryanair that is, just ask on the main ATC frequency blocking it for quite some time. METARs are always available via ACARS, same as TAFs.

Fuel and handling is ordered via ACARS, however, even then you can switch on the second set to the company frequency and afterwards back to 121.5.

...and on some you can use Box 3, the set assigned to ACARS, as a normal radio.

Ever heard of airports with no ATIS? Or the requirement to provide FIS? 🙄

The military had the right solution--the one radio has a "guard" receive only function. The pilot could hear emergency transmissions but would have to switch over to the main radio to transmit.