How can I implement such a cut-out effect?

Hi!
Tell me please, how can I optimally implement this effect with cutting the mesh?

this is not an advertisement, the game is not mine, it’s just clearly visible.

I tried to make modifications to the mesh itself. I just checked which triangle the point falls into and rebuild the triangle to make a cut, but it turns out too many triangles, since the movement of the cutting point would like to make smooth and it should work on mobile phones, as in this game.
I tried the option of drawing the mesh over and use the material through which the source will be visible, like dynamic holes, but I can not figure out how to make the sides of the cut.

Maybe there is some simple way to do it?

This video shows better

I found another one implementation. Works on mobile devices.
https://connect.unity.com/p/implement-sand-balls-mechanics

True, there is not quite a cutout, but it is possible to modify it.