here's how to change your material to unlit in unity !
if you are talking about the scene in sketchfab, the grass took all the poly count , lot of duplicate objects can be optimized by instancing geometry in ur engine of choice , can't reduce what is not reduceable (u need polys to make a shape) , it uses only gradient color palette texture , if i used some textures maybe i can reduce it but not it's not the case here,
this is already really low poly if u want lower poly u will maybe need to bake the textures (but not necessary at all , this is already really low poly) and i think u are actually talking about the sketchfab scene(it's the grass and duplicated objects)