Select one voice only in a closed score

That score uses only a single voice on each stave (even though there may be two people singing different notes). See https://musescore.org/en/handbook/4/working-multiple-voices for a description of how voices work in Musescore.

The two notes have been entered as a chord. If there were separate voices the note stems would be in opposite directions - up for voice 1, down for voice 2.

You can create separate voices using using the explode tool to separate the notes onto separate staves, and then using the implode command to combine those staves on one stave but with using voices. See https://musescore.org/en/handbook/4/implode-and-explode.

> How is that possible as there are clearly four voices?
Voices of singers and voices in MuseScore are different things.
MuseScore needs different voices if there are notes on the same staff but with different durations. You can see this if you select the measure, then each voice has a different color.
If notes have the same duration, they can all have voice 1 (or 2,3,4) (MuseScore) , like a chord. This means that there can be more than four notes on a staff at the same time, as is the case with some chords.
In your score, all the notes of the different vocal voices have the same duration, which is why everything is written in voice 1. You can do this, but you can also use several (MuseScore) voices.
You could also put all four in one staff and even in one voice, but that would be very confusing.