I know people have a hard time deciphering scenes, because people still don't get it even when spelled out for them, like the ssj caulifla scene, but trunks getting the energy really isn't so abstract that it's hard to get.
Trunks literally says to zamasu: "We will never let you crush our will to live".
Now prior to this, before the energy actually appears trunks says: "We work together and survive! That's who we mortals are!"
Right there is where trunks' willpower and desperation reaches its peak, you can even see it in how the expression in eyes change.
"Trunks' light" appears directly after this realisation; whatever happened to trunks is 100000000% driven by the power of will/hope/guts/spiral energy/whatevertropeyouwanttocallit.
Now with this trope it doesn't matter that's a bunch of "weak humans and a 2 kids", which is very reductionist on it's own but whatever, the point of this Power® is that it comes from deep within and is literally The Strongest There is, but only in deepest desperation.
So even if it was just 1 weak ass dumb kid, their hope and desperation would still fuel trunks to cut zamasu in half.
Also whatever XV did with the cut obviously isn't canon, it just serves the multiple timelines/universe spanning narrative of XV as a whole. Fucking GT was giving power to trunks SMH
