I am not aware of any tutorials about using Mega Voices.
As Ronnie has already said, rather than being played 'live' using the keys, they are really intended to be used in styles, MIDI songs, and Multi Pads where the specific note velocity to get a particular sound at the correct volume can be programmed accurately using step recording or editing tools.
You will find examples of styles, Multi Pads and possibly even MIDI songs in the presets on your S970 that use Mega Voices, so examining the MIDI events of these on the relevant channels may be of some interest and help. You can do this on the keyboard using 'Song Creator' for MIDI songs and 'Multi Pad Creator' for MPs, but 'Style Creator' is a bit more restrictive in what channels it will actually display in it's 'Edit' tab, so it may be better to use a computer DAW or a program such as Michael Bedesem's 'MixMaster' to look at styles.
Here is a link from which the program can be downloaded... http://psrtutorial.com/MB/mixMaster.html
An example of a preset Multi Pad on your S970 that uses the 'Trumpet' Mega Voice is 'BigBandSwing1' which is in the 'BrassPhrase' category.
If you open this in 'Multi Pad Creator' and select pad #4, then tab to the 'EDIT' page, you will see the notes in the MIDI event list that is displayed. If you move down the list to the first Note event, and then across to highlight the note Velocity in the column to the right of the note value, you can use the data wheel to change the velocity value and as you do so, the note will play the sound that is mapped to that velocity. By starting from 1 and working upwards, you will hear how the velocity values from 1 to 20 produce a trumpet sound played 'mf' which increases in volume as you increase the velocity. Then at value 21 the sound changes to the trumpet played 'f', starting out at low volume and increasing up to max. at velocity 40.
This way you can hear all the sounds for this voice all the way up to a 'glissando up' for velocities from 121 to 127. You can see the sounds and their ranges in the Data List, so this procedure may be useful to you in understanding Mega Voices.
Here is another link to a previous thread about Mega Voices which although doesn't expand much on the above, may be of interest. Although it refers to a PSR-S910, the example given in it also applies to your S970 since that voice is present on it too...https://www.psrtutorial.com/forum/index.php/topic,20103.msg137790.html#msg137790
Finally if you haven't already got a copy, it is worth downloading the 'PSR-S970 Reference Manual' which although doesn't help much regarding the use of Mega Voices, it contains information about using the Style, Song and Multi Pad Creators on the keyboard.